Have you tried the advice laid out for biting/nipping in Secrets to Dog Training? Theres a section that talks about the behaviour and might be useful.
Otherwise I'd say try shocking/startling your dog when he does it - make a loud noise, rattle some rocks in a can, spray him with a water pistol - just anything to startle him into taking a second to think "what was that??". Then, say good boy once he's stopped nipping.
Remember too, to remove access to whatever he's nipping at - fold your hands in front of you, step away - whatever you need to do to get his mouth off of whatever it is he's going for.
We have the same problem with a 13 wk old golden doodle. Wear jeans...and everyone has to immediately turn their backs on the dog and ignore it until he stops. Then praise and treat the dog. This usually works in a number of days but everyone has to do it and stick to it !!!! This has always worked with our labs....but this dog is different. And since she is already 13 wks it is harder too....we have only had her for 3 weeks. We are making progress but she has such periods where she is so hyper and then we have to immediately leave the room and turn our backs on her and then say sit. When she sits we praise and treat her.....and then return to the room. This is slowly working......but we have to be patient and persistent. A spray bottle, rocks in a can, etc don't work with her. Sometimes distraction will work but we are not as successful with this. Her bites hurt too....we have to stay really really calm or she flips out. Good luck
