My opinion? If your best friend is really your best friend then he is OFF LIMITS. You don't do that to a friend.It's wrong, and it would hurt your friend more than you know.
I know because that happened to 1 of my kids.He was 14. His girlfriend of almost 1 year dumped him for 1 of his best friends. It sucked, and i know how hurt and ticked he was So....and then they broke up like 2 weeks later! uggh He'll NEVER trust his friend again tho.Ruined a good friendship.
Sounds like the guy is just making the rounds anyway so he probably wouldn't be YOURS for long either. And then you'd be without a boyfriend AND a best friend.