Stargardt's usually affects central vision, so creates lower visual acuity (can't see as much detail) and colour blindness at later stages. It can also cause light sensitivity, which is why many people usually wears sunglasses. He should check with his doctor, red sunglasses can help out.

Has he started getting rehabilitation training? It would help him out to get it. Learning to do something in a different way may be more efficient than him trying to do things the way he is used to (for example, he may have trouble seeing the keyboard on his phone to type).

Depending on how far along he is, as well as how he is handling it, he may still be getting a grip on the vision loss. For that age, many people are trying to fit in and do what everyone else is doing. If all his friends are playing video games and learning to drive, he may not be able to. In terms of support, be there for him and talk to him about it. He may also want to talk to someone else who has vision loss, to see how they coped with it (but that doesn't always work).

Everyone's different. People ask "do blind people get annoyed by whatever," the answer is always going to be that some people do and some don't. Personally, for instance, I'm really hard to annoy, but then I used to be able to see so I kinda get your point of view. Different people, different experiences. All you can do is approach it as honestly as you can and understand that there's no way to guarantee you will always say the right thing. And that's nothing to do with anyone being blind, that's just relationships. So, maybe have a think about how best to react when one day you say something he doesn't like. Apologise and move on, usually, just like he should if he says something that offends you. Don't be a pushover, it isn't good for him or you.

In the end there isn't really any way to guarantee someone's going to feel okay about this stuff all the time. I don't know you or your friend, but I have spent a bit of time (as a sort of mentor) with some young people who are in similar situations to your friend. Something similar happened to me. I was a bit older at 19 but I suspect a lot of it will be the same. The only other thing I'd say is that rehab training can be spotty. Some people are great, other people are used to dealing exclusively with very young or very old people. I've only met three people so far think I get to work with people who don't fall into either one of those groups and find it really hard. There are alternatives to mainstream rehab training if the mainstream stuff seems too awful.

Honestly for my partner I'm sure he felt similar to you because of my blindness, But for many individuals as long as you're respectful, asking questions isnt offensive. It shows that you care and want to know how to support him. Make sure to stick through the bard times he has and just be there for him to the best of your ability. That's all you can really do :/