If he's lapping you, you should give room. Nothing is written in stone about it really, but the more aware and polite you are, the better off everyone will be. If I've got a big string of cars behind and I'm way in the back having wrecked or whatever, I'll just pull into the pits and let them all by.
By the same token, if you're involved in a battle for position, the leader/lapper needs to recognize that and pass in a spot that won't hurt anyone's race.