The two organisations you would do well to get in touch with and ask that question are as follows:
https://www.look-uk.org; and
https://www.victa.org.uk/

I know that the first port of call is typically the RNIB, but most blind people in the UK will tell you that they are rather ineffectual, and focus more heavily on perpetuating the stereotype of blind people as dependent and helpless. The above two charities I have linked do not do this. They are smaller, but their view of blind people is much more positive, and their advice will be better.

Hope that helps

My first thought when you said 'cancer' was retinoblastoma. If it's not, disregard this paragraph. Is the surgery the end of treatment for him or are they moving to other things? Is this trilateral? Radiation/chemo will affect brain development in small ways but if he's still in treatment, there are some immediate issues to address. I will list some that cross both the in treatment and tecovery/new life categories. If he's still at war, know that he's not alone in his campaign. Survivors (like me) are out here living our adventurous lives. My thoughts are with him, you and your family as you move forward.

Food. Not being able to see your food is a problem anyway, but if you're new to the genre as two year olds are, new things, different ways of preparing them or changes in texture or ingredients in familiar things can be issues. Use words. Describe food and let go of expectations for utensil use and eating compliance until he's has a chance to explore his plate. His hands are gonna be his eyes for a long time to come especially when food needs interrogating. Don't betray this trust. If it's a stewed tomato and you know he doesn't like tomatoes so you tell him it's something else? He will know when he tastes it. Eating issues will ensue. Chemo/radiation will affect taste, tolerance, appetite and acceptance of new/strange or various textures/temps, too, so even the most voracious eater will have compound issues after sight loss and during/after treatment.

Mobility. In hospital mobility is a Thing. Knowing your surroundings is important for everyone. For children with changed senses, it's imperative. Cane, walker or adult-facilitated exploration of rooms or treatment areas is psychologically necessary. Child life can or social services might be able to help you with this. I have a method I use for this I can teach you if you want. Any reinforcement of what little self-determination he has is very important within a treatment regimen. In non-hoapital or after-treatment environments, the option to explore mobility aids and choose which ones work for what environment are a great developmental assist.

Braille/raised letters/labeling stuff. If his parents want braille to be a part of his life, introduce it now. It's not the expectation that he'll start reading now but just the presence of cues, like any sighted child sees signs or has books around. Label things with braille stickers. You can use raised or magnet letters for this, too. I'm all about sensory integration so using a texture system for labeling things is another approach. (chemo particularly, will affect nerves and sometimes limits access to braille and can impair fine motor skills. Texture contrasts can get around this and help practice finding, aligning and mimic reading skills for when braille becomes possible, later.)

His brain will adjust to not having sight. Your job will be giving him the words and skills to use his changed perceptions without having to interpret them through vision he no longer has. Directions (right/left/etc, ) how things relate to his body (the table to your left/ three steps away,) textures, shapes and the clock face (for plate, room or table navigation) are very useful tools in and out of hospital.

There's an encyclopedia of other stuff but these things are a good start. We'll be here if you have questions. This group is full of very talented solution finders.

May your nephew's adventure be amazing.

So first of all, I'd like to say that this is probably the best time in a developing child's life to lose their sight.
They've already figured out walking (At least to some degree), they've provably got speech down at least a little.
I think the most important thing is to not baby them.
Don't treat your nephew like he's fragile, always encourage him to do new things, treat him like any other kid!
As with anyone, if you see him struggling ask him if he needs help but not force your help on him.
I would encourage you to think about how things could be more accessible, some tasks are needlessly complicated for blind folks just because sighted people only think about how to do it with sight, and just can't fathom doing it blind.

I don’t know if the U.K. has the equivalent of early intervention programs in the USA, but definitely get him signed up for that if at all possible. I would emphasize mobility- start exposing him to a cane (a light, tall one, the ball tips are too heavy for most small kids imo, and they grow quickly) and he might be willing to use an AMD to get around- I’ve seen kids use push toys like shopping carts as an alternative to more official mobility devices- what you don’t want is for him to be glued to his parents’ sides 24/7 and unable to explore his environment safely. He can still use most audio cause and effect toys, any letters on them can be brailled for early exposure, or use bump dots/felt/puff paint on any subtle buttons. Braille is not dead, it’s still important- there are organizations that will send free print/Braille books so he can feel the words as his parents read to him. I think one publisher also makes Braille/print board books which are mass manufactured. There are toys you can find designed for pre Braille or which aren’t specialized but work like texture sorting ones etc, block puzzles, and specially designed ones that help with letter recognition like the $1. Additionally, many blind kids are musically inclined, and if you find that, he would probably enjoy music lessons. Above all, like other posters have said, generally treat him like any other child- include him in cooking/baking/cleaning when age appropriate, give him chores, don’t let him get off scott-free for misbehaving. It’s definitely scary for him and his parents, but once that adjustment is made (and it may take awhile!) he will be fine. Support groups can be helpful, but meeting blind adults may be reassuring for them. Wishing your nephew all the best for his recovery and hoping for remission and maintenance of such.

I can't help with suggestions as to where to go because I'm in the US, but one thing I can make a suggestion for their organizations for later on when he's old enough that are I forget what the actual title of the big organizations are because I work with the smaller chapter I guess is the right word? There are companies that work with hiring blind people there's one called The Lighthouse for the Blind that's an Arkansas look them up, it's not specifically going to help you cuz it's in the US but see if they have something similar to them in the UK, they may be able to help you and your nephew very easily, especially if you find one that has after school services, like the job that I work at now, I can't name it due to privacy concerns as well as I don't know if I'm allowed to mention my job's name online due to potential non-disclosure I think? But basically we have a thing and they College the Red Schoolhouse that's not the complete title, but I can't give the complete title for the as mentioned reasons. So, they run an after-school program where they pick the kids up from school and they teach them how to read Braille, how to do things of that nature and also have to do how skills so they can be independent livers that's a can of worms I'm not going to go into because I don't necessarily agree with that portion but that's just me.

Link up with the TVI , teacher for visually impaired https://nystagmusnetwork.org/support-with-education-contents/early-years/the-qualified-teacher-of-the-visually-impaired-qtvi/

Join a peer support group for parents for now.

If you are in Northern Ireland angel eyes is a fab group

I agree with the other poster too, do not treat him differently to another 2 year old, let him run, climb up a slide, bounce on a trampoline etc etc

There are different opinions about when to start cane training and O&M but some kids that age can naturally gravitate towards using a doll buggy which makes sense as it will pick up on obstacles and steps etc.

Treat him like a regular kid as he grows up. Get him into audio books and braille as he ages.

A computer with NDVA screen reader when he gets old enough.

In the meantime, identify resources for the future and love him bunches now.

He can still hear, and laugh and giggle. Take him outside so he feels the sunshine. Park swings are good if allowed by drs. Help him experience life as he grows up.

Be there for his parents, be available.

My blind husband loves his Amazon Tap battery Alexa device. Its like the transistor radio was in the 1960s.

/u/lil_temple be the fun uncle/aunt. Make sure your nephew knows that you love and support him and that if he believes in himself he can go into any job, and let him know that it's his personality that counts not being "normal"

Make sure he learns Braille, buy him Braille picture books.

Make sure he has toys, I loved those books which made sounds, and any toy which made sound, and when he's a little older the Boppit series of toys was my favourite.

Just be in his life as a role model, go out with him and his family to wherever they are going and have fun with them, actions speak louder than words even for someone who can't see your actions.