One of my kittens — Nano, ~5-6 months old, got him at 12 weeks — keeps biting my fingers when being petted. It always starts with purring; Nano will seek me out at 5 in the morning, jumps into bed and starts purring like crazy, I wake up due to the sudden added weight on my shoulder/chest, and I pet him to say good morning. It can also be during the day where he will jump on my desk while I work, I stop to pet him and he starts purring.
He will then cradle his head in my hand, rub his head against it, and after a few seconds starts chewing. If I remove my hand, he follows and repeats. I hide my hands under the covers, and he walks over to my girlfriend and does the same.
I'm pretty sure he does this cause he's content, but it's starting to get annoying and as he gets older he bites harder. What can I do to stop it without discouraging him from coming to me for petting?
EDIT: He's not just nibbling on my skin, he grabs my finger (esp. my thumb) with his paws and puts it in his mouth.