Sounds like you have a lot going on. Take a deep breath and take one day at a time. Try to get to the bottom of his issue. It is human nature to think of the worse case, of course. I’m sure your little guy is picking up on your tension and fear. It will be okay. Lots of visually impaired children grow into functional visually impaired adults. Write down your questions and concerns on a notepad and take them with you to his appointments. Keep us posted.

I understand that you're under a lot of stress right now, and this probably isn't the advice you want to hear, but it's the advice you need to hear. It's already been posted, but it can't be overstated. The person you need to talk to about your questions and concerns is your son's doctor.

Did they state why they think it might be a tumor? There are a lot of different things that can cause blindness, some reverseable, others not so much

I don't want to discourage answering the other questions in this post, but I must clearly remind users that NO MEDICAL ADVICE is allowed on this subreddit. Do not speculate as to the condition they might have or what the treatment course or outcomes might be. No one on this subreddit is qualified to provide that kind of information.