Mom and dad keeping him home is not going to do anyone any favors. I don't see that being helpful.
As I said, I do feel for you in this situation. Transitions SUUUUCK.
But the only thing that makes them better is patience and consistency. I also suggest taking him full time for a number of weeks. I know you are cringing at the thought... But it seems like it would be a kind of immersion therapy. It does sound like things are getting better, albeit very slowly.
When it comes down to it; if it's not working, it's not working. If you are done with this, give the parents notice, fill that space and move on.
PS: He's not "suffering" !! He is just adjusting. The only reason it seems so awful right now is that the parents have not let their baby grow and develop. I know mom and dad SAY that they are doing the things you are asking them, but perhaps their idea is not the same as yours? The things you expect of him do not seem to be too much, and helping him move toward these goals is just that- helping him!
Hope it gets easier....