Bf changes his attitude towards me when he's with his friends.

My bf always has a mean attitude towards me when his friends are around. He won't be a total douche but he will act all wierd and different. Even with my younger sister(16) he'll get along with her really good when it's just us but when his friends come along he's mean to her . It bothers me because I don't like mean people. I've talked to him about it but he says that he doesn't act differently but he clearly does ! I don't know how to handle this anymore !