I think this is a very sensitive issue that cannot be fully addressed here. There is too little information about his treatment history to randomly throw out suggestions. Even if there is a treatment that has not been tried, it is really hard to force a family into a therapy process, if they are even able to do so financially or mentally in the first place.
I think the best you can do is to do your own research from an autism site such as the one S H listed and present the info to the family. But be prepared to be sensitive to their wishes for him should they decide not to take your view.
There is no treatment that is an easy cure..even successful therapies take a lifetime commitment to the process. Would you be willing to help commit to his therapy? Or just suggest that his parents do so? These are real questions that families deal with every day regarding autism.
Oh, and just for the record I am 100% pro-therapy, but I also know that the family cannot be forced into providing therapy.
Good luck.