Well, you are a part of his life, and he at least wants you to be a part of that.
Really, this is a situation where you both need to meet each other halfway. He shouldn't be trying to make you go out all the time, and should understand that sometimes it's OK for him to hang out with his friends.
On the flip side, couples are expected to do social stuff, and presumably his friends like you or at least want to get to know you better. So, hammer out a compromise. If he's worth being with, he'll work with you.