The time for a dog to learn what is okay and not okay to chew or nip is when they are still a puppy. Believe it or not, dogs have to chew, its in their nature and their toys are sometimes not enough to satisfy these bad habits.
To prevent your dog from chewing, start training him from day one! A good training method is like playing a game with him, the "drop it game." First, put your puppy on a leash. Second, place his chew toys around on the floor with objects that you don't want him chewing, such as toilet paper, paper towels, wallet, slippers, pens, books, etc. Then, let your puppy explore through the clutter, when he picks up his chew toys, praise him and encourage him to play with the toy. If he picks up what he shouldn't be chewing, give him the "drop it" command and jerk his leash (be nice). Try spaying your finger with the bitter apple spray and putting your finger in his mouth if he refuses to drop a object after you have jerked his leash.
Do this about three times daily, until he only picks up his chew toys. You can also use treats as reward, or give him lots of praises after each correct behavior. Remember if your using treats as a reward, quantity is not a big deal. Try not to give your dog too much treat. Breaking the treat in pieces usually works great.
To keep your puppy from nipping try the following method.
Sometimes simply getting more exercise can stop the nipping, we noticed this in our Yorkiepoo. Your puppy may have too much energy that needs to be burned off. Try taking your dog out for a walk on a daily basis or longer walks a few times a week. Find fun play games to do in the house like fetching the tennis ball.
We found that after we adopted another Yorkie Poo, the nipping and chewing decreased because they have each other to play with. If you cannot give your dog the exercise that he needs, try adopting another puppy or dog, they will give each other plenty of exercise.