How do you stop a child from wetting the bed if he's 6 years old?

May be normal. Up to 20 % of 6 year old wet the bed. If he has never been dry at night and he has no wetting during the day, it may be primary nocturnal enuresis. It also "runs" in families. Keep in mind he is not doing it because he wants to or has control over it. You may want to limit drinking fluids after 6pm, voiding frequently and not making him self conscious about it. There are also alarms you can use.