Do not punish a puppy for chewing, this how he INVESTIGATES the world, it's a NATURAL for young dogs to bite whenever their milkteeth has developed, and please don't kill the poor pup. Never hit your puppy hard or use strong acids on their mouth., And yes the previous comments are correct but to remind you, here are things to add up:
1) Use 1/8 mix spray of vinegar & water.Never use pure vinegar, the smell alone is very repulsive to dogs, this is all we need- the smell. You can dab vinegar or bitter apple on other things that cannot be moved like cables and even furniture if he bites.
2) Use the "conditioning law" by Freud. If the pup bites, use a strong "No", and if the puppy stops, praise him, give him a hug and a treat, puppy snack.
3) Crate train your puppy. If he cries, get him accustomed, do not give in. If he stops, praise him again, give him the treats - puppy snack time.
4) Keep all valuables and things that he enjoys away.
Remember, Lab puppies are always like this. They really love to bite, among the dog breeds. But they're very, very smart.