I agree that his share of responsibility isn't being met and that wouldn't be acceptable for me either.
He should consult with his doctor before stopping any medication, and act on his doctor's advice. But no matter how that unfolds, he should be capable of acting like a mature adult.
If not, you get to choose whether you can accept this behaviour or not. And yes, that's a tough decision to make, emotionally.
Good luck, hope something gets him moving.
Hugs