Indeed, as the above poster says, there's no real point in attempting to force him - all you can do is ensure that he properly understands the benefits of a "proper" grasp of the English language. How old is he, our of curiosity? I feel like in time, if he's a smart kid, he'll probably grasp pretty quickly that he's not going to get very far in life without being able to master even the basic "norms" of civilised conversation.