What are some methods I could use to stop my child from biting his fingernails?

Neg or positive. There are two ways to change behavior, negative and positive. For positive reinforcement we use rewards for any designated amount of time the child goes without biting. For negative we make nail biting a bad experience. One example is having your child soak his/her hands in vinegar. Then is tastes bad when they bite. It is up to you which you feel your child will best respond to.