I would definitely see a private behaviorist for this problem, if it's that serious, as we can only give advice based on your description... first-hand observation is much, much easier to help with. The Lab puppies/adolescents I have been around really didn't nip much, aren't they bred to instinctively have a soft mouth? Finding a good trainer would be ideal, to nip this in the bud.

I have heard that with very headstrong dogs, sometimes a low "no bite" that sounds like a growl will get their attention. I would think your other dog is teaching him not to bite? Within a week of Gonzo correcting Fozzie for biting, he completely stopped biting hands. I don't know exactly how old Kong is, but I know puppy needle-teeth are not pleasant.