I was a bed wetter. Bed wetting in the medical community is considered normal until around the age of 8. The Urinary Sphincter is not completely mature until then. I wet the bed till I was almost 13. It can also have to do with the part of the brain signaling you to wake up to pee not being mature as well.
My mother got the cloth pads that had the water proof backing. She would put a fitted sheet on the bed, the waterproof pad on and another fitted sheet over. So if I wet the first sheet there was on right under. You can also put a second waterproof pad between the first sheet and the mattress.
I wish I could tell you there was a magical answer, but it is natural for a child his age to wet the bed still.