Ok one more labrador question...My pup has his mouth on everything. From the time he wakes up....?

My lab puppy is 3 mo. and he nips and chews and bites from morning until night. Seriously, no kidding. I know from reading that the nipping is puppyhood and the mouthing is exploration and all...but we can't pet him AT ALL without his mouth opening up from the first pet. Is that really normal?? and how long does it last?