Well, does he know anything about transsexualism or will it be a completely foreign concept to him? If you want to know how he'll react, try mentioning it in idle conversation (or putting the TV on a show with transpeople in it or something) and talk to him about it... And just go from there (like if he doesn't seem freaked out, ask him, "How would you feel if I was trans?"). It kind of depends on if you've got a good relationship with him, too.
I planned for a long time how to come out to my mom and didn't even have to, as she figured me out first. My point being, these things don't always go like you plan, so just get him on the subject and talk about it. And good luck, dude.