"he's bagging groceries and living at a homeless shelter"
This will be a hook for you. This will keep you in delusion about him because the naive part of your nature, the codependent sensitive part of your nature, will think about that picture as if he were a poor homeless dog and not a calculating, full-on, selfish drug-seeking addict.
One of the important things I had to do regarding drug addicts was wise up to their real lives, not the one I made up in my head.
So I read some drug addict memoirs and suggest you do this, too, so that you can wipe away in your mind your vision of him as a lost, innocent young man who is hurting.
Nic Scheff's "Tweaked" is an eye-opener. I'd suggest that one for starters. He was a golden boy, in his family's eyes. But he was not at all who they thought he was. His father, David, wrote a book about the addiction taking over his son. It is amazing how his father continued to hold him on pedestal and think "surely not Nic" and all the while, Nic is living a life that is the lowest of the lows. And he doesn't much care. He loves drugs.
If your former partner was high all the times he was with you, then you had a relationship with an illusion. No one is himself when high. On stimulants he's powerful and erotic. On opiates he's warm and fuzzy. But he is never real. And everything is a lie.
You have been conned. It will be so important that you not make up a story about him based on your illusion about who you thought he was. If you see him for who he really is, then it will be easier to stay away. Because my guess is he will be getting in touch with you real soon. And he's going to throw you some powerful bait on the end of that hook.