Tad, sounds like it went pretty well overall.
I expect much the same sort of responses will be typical of my folks. My ma knows, in fact, she outright asked me and said she had always had a feeling. She said she's been working on my father and he doesn't really want to think about it I guess, but she said to know that no matter what he loves me. I expect he'll accept it, but not be as accepting as my mother, I suppose is the way to put it? Actually, I think he'd have a harder time if I were to tell him I'm bi than if I were to tell him I'm trans...haha
Maybe his suggesting you find out about your biological mother isn't so much him thinking it will fix you or that you don't know yourself as maybe thinking it might help you through the process? I don't know, that thought kind of goes a few ways. Like, since you've learned about, accepted, and come out about this maybe he wonders if knowing more "about yourself" (about your biological relatives) might be something you're interested in or that could help you. Or maybe, since these are changes you're opting into (the fact transgenderism isn't a choice is irrelevant to the fact that, at least in a basic sense, we opt into HRT and such -- we accept the changes they bring) he thought maybe knowing more about your biological parents might help with knowing what to expect in transition?
Uh, this is all speculative, of course, as I don't know how much he may or may not know, nor do I know him...but just some alternatives that are possible, I suppose. I think though that maybe if it bothers you that speaking to him to get it straight from him what he meant would be the best idea -- that way you're not left wondering or speculating or anything.