How long were you friends before trying to have a relationship with him? I had a best friend that we had never taken any further than that (besides a few drunken happenings) he had tried asking me out a couple of times but I had to be honest with myself and him that I didn't want it to ruin a friendship if we did not work out. He had always dated girls that were of the model type (small bodies, done up hair and faces) I am what they call a basic kind of girl I don't dress fancy, I don't wear make-up, and besides a brushing and a pony tail I don't do anything with my hair. So I felt that I would not be right for him in a physical sort of way.
It's basically just being honest with yourself in what is more important a relationship or his friendship? They also say that friends make better relationships. So in time once those past happenings can be put behind you who's to say that you two may not give the relationship another try. Right now be friends and stand beside him in the choices that he makes but it does not hurt to tell him how it makes you feel to be put in the situation of watching him hit on other women in front of you.
He still cherishes what you have between you two or he would not want to stick around to be your "friend" don't let that go, friends are a great thing to have specially of the oppisite sex because men are not quick at stabbing women friends in the back and you can talk to them with just about anything!!!!