I mean, it’s possible he was grooming you, but given the story you shared, I don’t think that’s likely. It sounds like he met you when he was still very young (a student teacher)… and in an effort to connect to students, he talked about things (like enjoying Joni Mitchell or candy corn).
I’m not sure he’s “innocent,” either. It sounds like he probably at least had a crush on you, but that doesn’t necessarily mean he ever intended to act on it, either. Then, after you’d graduated, this chance meeting gave him an opportunity to act on that crush, after you were legally an adult and no longer his student.
But I can certainly understand why, even after all this time, you’d feel confused. Especially given your own role as a teacher now, and also as a mother.
Still, it sounds like a fairly normal relationship. It sounds fairly healthy. There was nothing “dramatic” about it, once you got past the initial “I’m making out with my (former) teacher” phase, at least. And it seems that happened pretty quickly and naturally.
I guess my advice (as someone who had intended to be a high school teacher but my health issues meant I had to give up my MA just shy of 8 credits) would be, try to file it away as best you can as a relationship that was overall good. If you’d heard stories of him dating other former students, that might change that view. But I’m guessing if you had, you’d have included them — or at least have less doubts over the idea he’d been grooming you.
Still, as a chronic overanalyzer myself, I understand this is easier said than done.