i would check to see if his girlfriend is one of those over controling ones. He may really want to hang with you, but she may be controlling him to hang with her, therofore makeing him break a promise. So see what you can about the girl, she may be part of the cause. But in essence, a girlfriend should be a best friend with benefits, and should allow both people, to go hang with their friends, instead of just each other.
Simply, he should be able to spend plenty of time with his girl, and with his friends and brother.