Blondie:
You posted that he isolates himself, doesn't eat, doesn't work, doesn't communicate, etc., so sounds like he is not working any program. He is in his room using right? Not good for you or for him.
By allowing him to "flop" there and continue this behavior is not doing him one bit of good. What is he learning here? What do they mean "as long as he continues to go to his therapy?" What strides has he made? How long has he been in this therapy? I don't think these "experts" know he is still using. Although you said they are monitoring him? How so? Are they testing him? OMG! If they are testing him and they know he is still using, why are they allowing this?
Maybe I've missed something here!
I know how difficult it is to make your child leave the home! However, I'm sure you told him, before he came back home, the rules which were "no using" or out you'd go right? The first time he used he should have been out!
It is important that they know you mean what you say!! Just like when you discipline a child! You must follow through!
I did what you are doing for years and years. Don't be me, don't allow this to go on until he is well into his 40's! All you are doing is postponing the inevitable. Perhaps it's time to take a stand and mean it! Just keep saying to yourself, "it's for his own good!" Making him confortable and allowing the same behavior is not in his or your best interest! The only way he will get better is when he knows he has to help himself! The choices are his.
My son didn't get better until I finally leg go! I spent 35 years as a co-dependent!
Prayers heading your way,
Devastated