Hi, I'd like to help you with your question.
If you have had several incidents of him doing this in your relationship, then either he feels his friends are more important than his relationship, he is immature and doesn't know how to balance his friends and relationship, or he has an issue with you and for some reason is not willing to talk about it.
You can talk with him again. Sit down and bring up the topic in a neutral, non accusatory way. Say you want to work it out. Ask him for ideas on what he thinks will work. Ask him about issues in your relationship that may cause this problem. When he talks, try not to get defensive. See if the two of you can work out an agreement and stick to it.
If he has no interest in working on this issue or tries to get out of the conversation, you may want to rethink the relationship. As long as he avoids the issue and keeps up the behavior, it is not going to get better. You may want to decide whether or not you are willing to be in a relationship where you are second to his friends.
I hope this has helped you,Kate