I've been where you are, and I know it's no fun. The thing is that your son is going to develop his own relationship with your parents, without your oversight, whether you want it or not.
I hated it when my son met my mother without me. I worried about what she would say and what he would think of me. But then I decided to let go of it. I decided that our relationship was strong enough at that point in our reunion to withstand my mother's input to my son. They're big boys, and they have a right to know their grandparents if that's what they want.
When my mother met her grandson for the first time, I wasn't even invited. They met at my brother's house, and to this day I have no idea what they talked about. It turned out that I really had nothing to worry about ~ my son can't stand her pushiness and manipulative behavior. But I do remember feeling incredibly hurt that I wasn't invited. The weird thing is my mother stopped by my house on her way there to give me some really nice fish from the fish market we went to when I was a kid. Strange....
