That’s a really tough question. Note: I’m using gender specific terms to make a point.
From what I’ve seen most TGs born in a male body whose feminine side is strong enough that they want to dress, grow breasts, or what have you will go one of two ways: [1] they will either progressively go trans; or [2] they will scare themselves sh*tl*ss and turn ultra male-macho. Most of the cases I’ve seen go the former route. But, then, most of the cases I’ve seen come from associations with the transsexual community. Either way, you have to remember that he’s discovering himself for better or for worse. What he says now may not be what he says 2 years from now, as the issues involved are complex.
The part you need to think about is not “what might he do next” or even “what is he willing to discuss with me”. What you need to consider is what makes you comfortable and happy in your relationship. You cannot expect to mould him into something that he’s not. If you try to force him to fit something in your mould you’ll only drive her away. But how much of her can you accept, respect, and enjoy? That’s the where the proverbial rubber meets the road.
Relationships are much more complex than we generally like to believe because people idealize about them. Relationships are not only about sex or support or trust or comfort or whatever. Relationships are about ALL these things. If one of those things is broken everything else suffers with it.
My advice is rather than questioning him you should be questioning yourself. If you are unhappy—you being half the relationship—then the whole relationship suffers. Just as if he is unhappy. You have no control over his thoughts, feelings, desires, or actions. You only have control of your own. A relationship, whether healthy or not, is based on the interaction of both of you being responsible for your own input.
If you cannot be happy then get out. Let her (him?) know why so that she can deal with it all in an honest way. If you think you can be happy figure out what you have to do to reconcile the things that bother you. Again, your happiness is your end—he is not either to credit or to blame for that.
Lastly, no matter what you decide to do, be honest. This will prove to be healthier both for you and for him. Whether that honesty is supportive, pejorative, or just plain lost, your ability to be up-front may make all the difference to both of you.
~~Steve