AmysAnimals: I think you should listen to Foggy. You can do a preventive mites treatment (at home or consult your vet) just to rule it out. He can be a temperamental pig anyway, but at least you'll know he is not in pain. Read this:
http://www.guinealynx.info/forums/viewtopic.php?t=30325
I had a similar problem with Linny. I still don't understand why he would enjoy being with my daughter so much and never bited her, and why he would bited me so hard that made me bleed.
With the others I would say no when they nip my fingers. Chibi used to do it more when I got her. With time she stoped. You say he is being with you for 3 weeks. My piggies needed more time to get use to us. Penny (since march) and Chibi (sice july) are use to us by now, but Chimicuino (since july) still needs more time.