Oh Baby Girl ~ (sigh)
Just say no. He wants his cake and wants to eat it too. Life and relationships don't work this way.
If the tables were turned and you said this to him, would he like to think of you with another boy in your arms, all the while he is waiting his turn to be with you? (yuck either way) I would be willing to bet, um, NO. And he would call you bad names for it on top of it all. Intimacy is diminished if it isn't exclusive. That is the very bottom line.
Having friends of the opposite sex is one thing. Sleeping with them is Really something else. Don't let him take the specialness of this away from you because his hormones are raging out of control.
He is a young man who is full of being a young man. He hasn't learned some of the basic affirming behaviors of being a boyfriend yet.
Stand your ground. Sorry to say, that being so young, (I know you don't want to hear that) if he wants to experiment, he will.
(As an aside ~Don't be intimate with him if you haven't already. HPV is a permanent problem that will not affect his health but will affect yours. It is epidemic.Just sayin')
P.S. You are no doubt a Knock Out if you are his girlfriend! Don't ever feel bad about yourself because of his bad behavior! Many Hugs to you.