Stepson

My 6 year old step son is doing horribly in school- getting on bad "colors" on a very regular basis and got a bunch of "i's" on his report card. My husband and I do not tolerate this kind of behavior but his mother lets him do whatever he wants. We tried to go get him today from his grandmothers house (after letting the mother know) to discuss his report card and behavior. When we got there, the 6 year old had decided that he wasn't going and would not come out of the house and the grandmother wouldn't make him leave. She actually did the opposite and protected him, as if we were going to hurt him in some way. I'm so frustrated with this entire situation and I just want to give up. Education is so important to me and I want good things for my stepson. Does anyone have any suggestions? I want to take Christmas away from him but that may be a little harsh. We've always told him that kids who make bad choices don't get presents. What do I dooooo??