He already said, from what you reported: "When I tried to reach out to him once back in August for advice he told me that he couldn't talk to me about anything related to my therapy/treatment since he was no longer my T" once already.

You can send that friend request but, my advice, is to prepare yourself for a potential 'no' to said request. Why? It is different chatting with him when you casually meet but 'friending' him is another level of intimacy.