Wetting the bed is common in children and is something they can't control.
Limit his fluids about an hour before bed and have your child go to the restroom to urinate 2 hours before bedtime and right before he goes to bed. This will ensure his bladder is empty which will leave room for more urine.
Set an alarm for yourself to get up and then go wake your child and have him go to the bathroom.
You can also get some bed mats to put down on your child's bed to protect their mattress from the urine.
If you can get him to wear bed wetting diapers then he would be more comfortable in them.
He might also fit into Pampers cruisers size 7 diapers or even Parents choice size 7 diapers.