How do I stop my kitten gnawing?

[MeredithJay]

The biting isn't violent, it isn't even playing. He has his litter mate to do all his violent play with, but when I'm sleeping, and my foot is sticking out from under the covers, he'll just bite it. Never hard enough to draw blood, and not with much force, he just has razor sharp teeth, so it hurts. Also, when I'm sitting in a chair reading, he'll come sit in my lap and purr purr purr, and then bite my arm. I think it's affectionate, but it's really quite painful. Is there anyway I can get him to stop?
I've tried just ignoring it, or moving so his mouth can't get to me, but that's inconvenient, especially if it's too warm under my covers.
I'm hoping he'll grow out of it, he's about 4 and a half months old right now.