How to make him believe

Just tell him you're telling the truth. If he can't accept it, then what can you do? That's that.

have you lied to him before? then why would you lie now) if you have been together a while he should believe you, the other people are just jealous

You're at a very young age and people your age like to talk a lot of crap. Tell your boyfriend if he can't trust you, then you guys have nothing. A relationship is all about trust. You have to be able to trust each other. It gets harder when you get older because you're supposed to grow out of that kind of immaturity, but for some people it just intensifies.

Try to be mature, and if this is causing too much drama, just move on. You're probably too young to be taking dating so seriously any way.

Well if he really loved you, then he would have believed you more than anybody else..but then if he didn't believe you then maybe he was looking for the right thing or if to dump you. But sooner or later he will be running back. Just go talk to him about it tough, just hang in there.