What's the politically correct term for the retards with big heads?

Asked this earlier and didn't get any real answers. I have a friend Jim who's a retard. It looks a lot like other retards you see on TV. It has a big head, a flat bowl cut (like Moe Howard) and is a little chubby if that helps. What is the politically correct term or medical condition for what it has? I know there's a better way to describe it but I can't figure it out