I want to tell my boyfriend to not hang out with his best friend.
Why wouldn't you want him to hang out with his best friend?
I think the guy is a jerk.
You sound very controlling. You shouldn't prevent someone from hanging out with their friends. You could put him in a position where he'll have to choose.
That might just do it.
Why don't you just tell your boy friend you won't hang out with him while his friend is around. That way he can keep his friend and you don't have to deal with him.