just my 2 centsSeeing that you are seeing a therapist, not sure if any of these ideas/comments will be new but here it goes.....
if you are using diposable, try switching to cloth
put him on the scale and weigh him, and them tell him that the two of you have abig problem...that no one makes diapers in his size, and ask HIM what the both of you should do. MAybe he can come up with a solution all on his own.
Just an idea.