I can understand your saying that he was never high but the fact that one has to go through rehab to get off any drug at all states there is a major addiction, even if not causing an altered state or high. Thing is methadone is known to be highly addictive opioid drug so some effect on him has to be the case.

A drug like depakote that I was on for over 10 years had an effect on me, and although I was never "high" on it, I definitely am a different person now that i am drug free. I was not "me" on it and it's part of why I quit. Just an example

I cannot say he'd never come back with any kind of authority. But i can say you need to move on, if only until he chooses to do so. You have to, for now, decide that no one, not even someone you love and care deeply about should have the ability to stop your life from going forward. If someone loves you they would not want you to pine away for them when they can't or won't be with you.

Also, having been on a strong drug like that, again not considering being high, internally something has to have changed. If it had no effect on him he wouldn't have been dependent on it. So perhaps part of what he's feeling is the rawness of facing life without this life-changing drug. Maybe it's just been too much for him to bear at this point of time in his life.