That's pretty normal behaviour, sometimes they'll do a little test bite...the only times my rats have done it is if I've been feeding them through the bars of the cage then stick my finger in and they try to grab it, thinking it's another piece of food, or if I've stuck my finger inside the opening to their "house", I assume they do it then because I've startled them...or maybe it's the food thing again *shrugs* And they seem to do it more when they're younger, and might be testing their limits with you...
But the point I'm trying to make is that I've never had this behaviour lead to real biting, and they "grow out of it" for the most part (with the exception of the food through the bars thing)
If you don't like that he's doing it at all, I've heard that if you make a squeaking noise everytime he does it that he'll eventually stop doing it (because he's treating you like you're another rat, and if you squeak then he'll think he's hurting you and stop) I've never tried this, so I can't say whether it works or not, but I imagine making any sudden noise when he does it would make him stop (negative reinforcement and all that)
Long post for such a simple answer, eh?
By the way, he'd be what you'd call a "blue" rat, a blue berkshire to be more specific (berkshire being the term referring to the white patch on his belly and his white hands and feet)