Why does my boyfriend never ask me to hangout with him and his friends?

Me and my boyfriend have been together for 8 months now and he never asks me to hangout with him and his friends. I go to school with most of his friends, so I know them and I talk to them. They always ask me why I never hangout with them and when I tell them he doesn't invite me, they say he tells them he asked, but I said I didn't want to go(which is not true). And most of the times if I invite him out with my friends, he doesn't come. I don't want to like invade his personal friend time, but it concerns me when he goes to parties with his friends and doesn't bother to invite me, especially since all of his friends have boyfriends/girlfriends and they go with. It's not that I am embarassing. So why do you think he never invites me?