Op l feel for you, my dd doesn't have sen, but was diagnosed as having a fairly severe joint problem at 3 yrs old. It is not life threatening and will very slowly ease once she is fully grown.
She couldn't do lots of activities, had to sleep every pm, couldn't go to most parties , do many sports etc.
I was often up all night comforting her as she was in so much pain, it got a bit better, until recently when she is going through puberty which has set the pain off again, she has missed 9 week of school this term and l have not slept much helping her.
The last 4 weekends have been completely taken up helping her catch up with schoolwork, and tucking her up to rest in between.
The worst was at primary age most parents just thought me overprotective as her issue is not visible, and due to my careful structuring of her life she mostly gets to school.It has been hard and lonely at times......BUT l spent some time with my DS at Great Ormond Sreet for a small op he had at age 1...and it completely cured me of my sadness for my DD...l saw so many children who will not live to see 13 (the age DD is now) with life limiting illnesses, and the struggle on their parents faces l will never forget.
So if at 3 am, l am massaging my DDs painful knees, worrying about how much schoolwork she will inevitably be missing tomorrow, and how tired l am going to be again, l remember those kids and parents and kiss my lovely DD, so grateful l have her, l hug her and tell her what a joy she is to me.
Lastly l never let her become self negative about the things she cannot do, l won't let her be a victim of her condition, or wallow.
I remind her there are other kids far far worse off than her.
Love your Ds as l am sure you do, let him (and you) walk to the beat of your own drum...do it your way and be proud of him.