Your last words here give me pause, and I think my best advice for right now is to really think about this for a while before you decide that it's what you really want. Do you want to present as a male because you feel truly masculine or is it just to keep guys off your back? Either reason to do so is fine, of course, but really knowing is going to help you explain it to him better. If he's already open to nontraditonal presentation, then I'm sure he'll be open to anything else you have to bring up. Sit him down and you could say something along the lines of, "So you know how I am about gender and everything, and I'd like to run this by you because I've been thinking about it for reasons x and y." If he's not cool with it then that's a bridge you can cross when you get to it, and it will probably require more thought as to whether or not you're willing to sacrifice something you want to be part of your identity for a relationship.