I work with people with developmental issues, and one of them has pretty severe autism. I've taken him out for day sails in my Crown 28 and he has enjoyed it immensely - but I don't do anything like that without another staff member who could handle him if things get out of control.
I'd say make it gradual - add it to his routine. Don't go out for a sail until he's been to the boat with you 10 times, and then just go out for an hour. Figure out things he can do to be a part of things - totally minor things can be huge and help to deal with anxiety (i.e. I always put the mainsail cover away).
In many ways, the careful order and simplicity of a well-maintained sailboat can be a fantastic place for a person who struggles with complexity and change in the way a person with autism can. I'm not autistic, but my boat is much more orderly and coordinated than any other aspect of our lives.
If you manage it carefully, you might well be able to give him an amazing gift. But be careful.