Hey, thanks for coming by.
It sounds like you have a relationship wherein you are both open and honest with each other (which is optimal). I don't see any reason for you to worry that he might be attracted to men.
Based on my experience (which obviously is not his, but it is what I have to go on), I am transsexual, and I am attracted to women, not at all to men. I believe most "cross dressers" are attracted to women. And, most transvestites are attracted to women. As I understand it, it is when you change the subject to transsexuals that the number shift substantially (substantially more are attracted to men). I have read that about 1/3 of MTF transwomen are attracted to men before transition, while nearly 2/3 are attracted to women. After transitioning, I have read (don't remember where) that many people's attraction shifts, and more MTF's are attracted to men than women. But, still a large percentage are attracted exclusively to women.
So, even if he some day decides that he needs to transition, there is a good chance SHE will be attracted to you still. And, you might find that to be better than you might imagine. It sounds like the two of you are adventurous.