As someone who has kind of been hurt in a LTR because my CDing didn't fly well with her, let me first say that I think it's awesome that you're so accepting of his habits.
That being said, sometimes accepting and running along with it when you know it's going somewhere you're uncomfortable with would be more detrimental to him emotionally than to just be up front now and lay out clear boundaries.
From your brief description of the way he talks about transitioning it sounds like there's a lot more information underneath the surface that he should be telling you to help you decide if this relationship is right for you.
So I'd suggest a more proactive approach. You don't have to be mean about it but I think you should tell him honestly how you feel and your concerns about being a bridge. If you would leave him if he became a woman, TELL HIM NOW. That way you don't spend months in a relationship waiting for him to decide to transition and then leave him feeling betrayed. If you suspect you might be a bridge, TELL HIM NOW. From personal experience, I really really really strongly suggest laying out your personal boundaries now. That way, if he knows he HAS to cross them because of who he is, he should be honest and forthright enough to come out and tell you that he doesn't think it will work between the two of you.
That's really the only way to get him to talk. If he likes you and wants to keep you he might not tell you "dealbreakers" (hoping that you won't care in the long run) unless you come out and say "here are my dealbreakers."
