IB, I am so sorry your xh and the ow felt a need to be so classless. Sad, really.
I am sure your son was upset. Unfortunately, as much as we want to, we cannot do anything to fix that for our children.
I agree with Punkin. It is now up to your son to tell his father how he feels. He will not hear it if it comes from you. And he might not hear it from son either.
We should never, ever be defined by our roles. Rather, we should be defined by the kind of person we are. You are a strong, loving, loyal, intelligent woman.
As far as your ex and his antics - You belong there at your son's games. You cheer him on and act with dignity and grace. Believe me, people are shaking their heads at the two of them.