Leaving it up to him is definitely the best approach. Especially because, well, he might not be trans at all. Knowing trans people is always cool, and being able to talk about your experiences is great, but some people are stealth and would like to stay that way. I'm stealth, and while three of my good friends are trans guys, I'm only out to one of them (and that's because we went to the same trans support group for a few months). Even when it's trans people asking, I have no interest in telling them I'm trans. Like Tossu said, I'm just a guy with a medical condition, and while some people are perfectly happy talking to others about it, I'm not. If the hat shop guy is trans and wants to discuss it with you, rad! If not, it's better to leave it open-ended on his part than to bombard him.