Undicisplined player

I have a troublesome kid who can play great when he wants to but is often unmotivated and totally ignores his blocking responsibility or simply doesn't fire out and block. He'll just stand up and let the defender blow by him. Other times he can pancake anyone you put against him. We have him playing both ways on the line and I know he loves defense and doesn't care to play offense.
Tonight in practice he and the quaterback got into a tiff comming out of the huddle so he let his man blow by him untouched to the QB. I immediatly replaced him with another guard and instructed him to play deep saftey and also told him he can only two hand touch any runner who reaches him............this kid loves to tackle. My reasoning was if he refuses to participate on offense I won't allow him to particapate in what he wants to do. I talked to him about team concepts and explained that he has to make scrafices in order to help the team. It didn't seem to help.
Any suggestions will be appreciated.