I have some back problems and I cannot stand for long with out pain, so being the keeper of the club lathe I made a lower stand for it so that I could sit and turn, just as I was thinking of a project for me to try it out, my son-in-law turns up with a seat for me! I find the seat is the better idea for me, as turning seems to involve a considerable amount of small body movements, almost a dance routine at times. Constantly moving the stool means getting on and off takes a lot of time, but gets the job done.
There are two things to consider when setting up for the lad, one the level of his disability, and two what suits him. You will need to consider what suits his personality as well as his physical ability.
You could do well to look on ebay and get a drill powered lathe and make a stand for this and then he can 'have a go' and if he takes to it then go for a more substantial arrangement.
Good to see that you are trying to help the lad and see the person and not the disability
Ron