I understand how you feel, and i would be pissed if my boyfriend kept canceling our time together to hang out with his friends. I think you both need to sit down and give each other feedback on what you want out of the realationship, why you love each other, and something you wish the other would change. I promise it helps. My boyfriend and i do it occasionally and even though it can hurt sometimes, it helps. Just make sure you dont take all the time away from him and his friends, and if his friends are doing something on a night you and him have plans, let him go and see if you can go with them.
hope this helps