Puppies love to play, but they don't always know how to do so. Your puppy is used to playing with his mom and litter mates, so when he comes to his new home, he may assume you are his new playmates. While you may enjoy tussling gently with him when he is small, the bigger and more insistent he gets, the less amusing it will be. Fortunately, most puppies are eager for attention and, if you take the time, will quickly learn what is acceptable behavior.
Teach your puppy what is allowed. If he starts chewing on your fingers while you are petting him, say "ouch" and drop your hand. Don't jerk away suddenly, as that will trigger his prey drive, further exciting him. Wait about 20 seconds and resume petting him. Repeat a few times if necessary. If he continues to chew on you, it's time to find something else to do -- he needs to work off some energy.
Provide toys. A variety of chew toys, including hard rubber toys and twisted rope bones, provide plenty of textures for your puppy to mouth and explore.
Give your puppy plenty of exercise. While it is fun to cuddle your pup, he needs to move around and explore his world. A tired puppy will lay in your lap and enjoy pats rather than gnaw on your fingers and pull on your pants legs.