If it helps, a trans guy I know has fully transitioned and kept his boyfriend from when he presented as butch femme and even as girly, so it is possible. I guess it does depend on your boyfriend's sexual orientation, if he is 100% hetro then it nay be very challenging for him. If he has some room to move sexually in the bi direction it may not be as big an issue as you fear. Keeping in communication is the most important thing. He should see a counsellor if there are things about himself that he needs to discuss for professional support. As is always the case with these things, you don't know until you know...the only way is forward. 🙂