First of all
Dont immediately think you are doing something wrong
Look at the situation
His irriatation comes frome something that happens in the little group steeing.
Have you sat on the sidelines and watched what they do or dont do?
Do you talk about wha the likes or does not like when he is wiht them?
Do you realize he is finding his own way to deal with some of the condlicts we wil go through in life, and a simple reinforcement from you and your husband may be all the support he needs.
If the frustration becomes anger and violent, then there must be a fix.
If he is a bit crabby, find ways to bring him back to calm awithout slamming the other children as people.
Tals about actions, not individulls.
My daughter had a friend that always had the latest gizmo even though I felt she was too young to deal with most of them.
I chatted with my daughter about what I didnt like and what I would or would not do in the situations that came up. I also explained what my husband and I expected of her as a duaghter in our family strucure.
In the end,,,the other girl has moved away, lsee choas in the lives of the girls here. Mya daughter even mentions how she is glad we didnt give in to all the whining she did when she didnt get everything she asked for. She also reminded us that she would continue to whine for things. :)
Alll in all...You have to do what is right for you as a parent.
Sometimes we eant to keep them away from all the stuff we dont like, but we have to teach during those moments and not jsut set up road blocks and detours.
Watch, use the information to teach your son, and remember that he is young, but he wil listen.