I feel like someone else on the forum was in an eerily similar situation and pulled the plug, but I can't remember who. Maybe if they see this, they'll have some firsthand advice.
He has talked to doctors and has been on medication before. We actually have been off/on friends for the last 10 years (3 times we have broken off and not spoken to each other) but he had a different best friend (also love interest) who is now completely out of the picture. (Another high school friend.)...... You have a codependent leech, not a friend. He needs serious help, but I doubt he'll seek it out, based on what you've said
He does have other friends, but he dislikes them. He wants more friends, but also thinks he has more than enough and doesn't want to bother with making more. It's an endless cycle with him being a-okay, then blaming me, then hating me, then dusting off his shoulder and being fine again, expecting me to just brush things off too.
Also, he thinks I don't believe he had an illness. He keeps telling me I can talk to his doctors and they can explain that he is bipolar, depressed, lonely, borderline suicidal, etc...
I know I have the freedom to walk away, and that I shouldn't feel responsible for his actions, etc... it's just hard. And I know it shouldn't be, given the way he acts, and in the end I'm really only holding myself back by making excuses for him. :-/
Bleh.