Well, obviously, your boyfriend isn't trusting.
If this "friend" has a "friend" and he doesn't have any information that you could easily get off anyone at school, then that is pretty much no proof that anything is happening. The fact that he won't reveal who this person is also shows this.
When I was in school, I could get just about anyone's number/location by making only three phone calls, max. I'm sure, with the way kids use myspace/facebook/etc. that it would be even easier to get that kind of information (unfortunately) so just having it doesn't imply anything, and your boyfriend is being immature to doubt you just because of that.
Also, information passed on like this is very unreliable.
For example:
Friend

tells friend: Guess who's number I've got... [Insert your name here!] *giggle* as an immature joke.
Friend tells boyfriend: Guess what? Friend

is calling your girl!
Boyfriend thinks: My girlfriend is cheating on me with friend's friend, Friend

?
Talk to him about it, and if he fails to see the stupidity behind these claims, then I wouldn't think he is right for you.
BTW: How old are you? Middle school, high school?