Ever hear people talk about how they want the love of their lives to be their best friend? He probably means that he feels like he can really trust you and just overall loves you. Best friend and girlfriend, that's a pretty good combination I would say. If he was lying to people and not saying you were his girlfriend that would be cause for concern, but this doesn't seem to be the case.
it just means he really really likes you. And he probably want you guys to be together like that forever. Best friends are forever + he says you're his girlfriend so ofcourse you don't hve to worry about just being a friend.