My son grinds his teeth at night while he is sleep what can I do?

Normal. A lot of kids grind their teeth. Most likely this is normal. I don't know the age of your child, but you can always have his first visit to a pediatric dentist at this time.
Depends on age. Lots of kids w only baby teeth grind their teeth because 1. Jaws grow in different spots on different days so occlusion has to be ground in, and 2. They learn so much during the day that they back up the "computer" @ night. Night guars for kids are overkill. If over 7 and still grinding, see an orthodontist to check for malocclusion.
Grinding. You want to have him see a dentist to evaluate potential dental issues that contribute or cause the problem such as his bite and other things. He may need to have them treated. A bite guard either customized or a regular one bought over-the-counter may help sometimes.