How do you ask?

[BrendaPDX]

Just wait and see, and does it really matter? We always want to share

[Ressie]

"I thought it was so cool when you dressed up like a chick on halloween that one year." "I'm thinking about doing that this year too, in fact I already have a wig".

[Sarah Doepner]

I'm with the folks who suggest you decide to tell him about yourself first. If he wants to share with you, fine.

When you feel confident enough to do this you could go to him and say something like this. "I respect you and trust you. I'm looking for an ally and you seem to not be ashamed that you dressed very convincingly as a girl on Halloween once and seemed to enjoy the experience. I'm transgender and if you have questions or reservations I'd like to talk about them because you are my friend and I don't want to damage that relationship."

Or something like that. Be patient. You may have the opportunity fall into your lap some day, or you may decide to never tell him, but it doesn't show respect to put him in a corner like that.