My 3yr old son grinds his teeth when he sleeps, how can I get him to stop?

ENT. Take him to an ENT and have his airway checked. Grinding helps open a blocked airway.
Check tonsils. Some people, big or small, will grind no matter what and sometimes that is genetic. Others grind because of a condition in the mouth, like a high cusp, or just bad habit. Many people, including kids, grind because their mouths are too small for their tongues, so that nighttime breathing is a problem. Have your pediatrician check tonsils and adenoids which may be enlarged from infection or allergy.
May not. You may not be able to get him to stop. Depends on the cause. Here's a useful link for more information http://kidshealth. Org/parent/general/teeth/bruxism. Html. You should also take your child to the dentist.