Okay. We need this clicker and some training treats asap. My arms can't take the little baby teeth much longer!
When he nips, gently circle his muzzle with your hand and hold his mouth closed. Say, "NO chew - just kisses!" When you release his mouth, he will usually involuntarily lick his lips. Then you say, "GOOD kiss!" Being Labs, they have to use their mouth somehow . . . so you redirect the chewing to a praised behavior - kisses.
That's how I trained my Lab to "kiss" instead of chewing! It actually is funny, because if you ask her for a kiss she will give you a tiny little lick on the chin or the tip of your nose . . . I find that preferable to the "Full Facial" that most Labs dispense!