Hello and welcome to cruise-chat.
A good thing about the Adventure is that it is so easy for those in chairs to get around. A friend of mine who has to use a wheelchair went on the Mariner last year and had a good time.
The crew works well with disabled passengers, and your son will not be the only one in a chair. Every cruise I have been on for the past few years has found chairs in the passageways plugged in for recharging.