I would be just as critical as you OP but I think you have to leave it alone. He's not your kid it's not your business. You can model better behavior, you can try to encourage better manners from him when/if you interact with him, but you certainly can't "intervene" without almost certainly creating tension (potentially significant) with his mother.

Teach your child differently and focus on that.

I learned manners from my aunts. My parents were oblivious. Go ahead and correct him. It's ok to be restrict aunt.