My best friend's son called me "mommy."?

Well, from what I understand is he sees you as more of a mother figure then his own mother. I don't think it's really a problem as you have been in his life since he was small. Just have a serious conversation with your best friend and tell him, if he's not ready for a relationship... then your willing to wait. As long as he is strickly monogomus with YOU and only YOU. Also tell him that your not sure if his son should be calling you "mom" if YOUR not even sure what the status will be with your best friend. I hope this helped.