I have a dog with a annoying habbit?

I have a 1yr old pitbull he's a great dog we've had him since he was 7wks old and have worked very hard with him,he's been socialized with people,other dogs,he knows fun tricks,and knows basic commands,sit,down, leave it,no,back up(when standing in a doorway),come,mine,and stay,The habbit he has is when you pet him he likes to nibble on my fingers it's not all the time but I would like to be able to pet him with out him wanting to nibble ! I don't know if it's a playfull gesture or a breed thing ? when I tell him to stop he quits for a minute and then right back at it!We do have chew toys for him he likes(just not fingers) LOL Any way if someone has any helpful suggestions that would be great! Thank you!!