I was going to raise the same issue of how it's not really fair to him if you're going to have to come out to him later, anyway, but if you're not planning on transitioning.... I guess it's simpler. Just be yourself. Seriously. There are plenty of guys who like masculine girls, acting feminine is definitely not the key. I stayed far away from straight guys before transition, but still got some flirtation directed towards me, as masculine as I was. Keep in mind, any part of yourself that you change to attract a guy, you'll have to suffer keeping up the illusion for the duration of the relationship or change it back to normal and risk him getting upset about it.
Try some general, gender neutral flirting. Small smiles, compliments, light physical contact (like touching his arm casually). And somehow make it clear to him that you're not a lesbian so that doesn't make him keep a distance of just friends.