In my opinion you cannot change someones sexuality as what they are sexually excited by. His sexuality may be unformed and undiscovered but your words leave me thinking otherwise.
Ask yourself if he looks at you like other men have when seeing you naked. Do you see the hunger in his eyes and feel the heat of his sexual interest in you and for you?
Often people will reject what they are naturally attracted to and try to force their sexuality to conform to what they think is the correct way of feeling and behaving. This causes misery for everyone.
He is not rejecting you and I suspect he would be acting this way with any woman.
Try to get him to open up about his darkest fantasies because usually that is where you find the actual persons sexuality.
Also explore possible sexual abuse because abuse will twist the natural expression of ones sexuality as they try to resolve and make sense of the original trauma by repeating it.
Ultimately don't compromise your own needs and wants because you will not be doing either of you any good in the long run.
You may want to read about Crossdreamers by Jack Molay.
