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!
Thanks. This is great first hand knowledge information you're giving us.