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

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.

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.

ENT: Take him to an ENT and have his airway checked. Grinding helps open a blocked airway.