Getting Urine Off Memory Foam Mattress

My son is still a bed-wetter and he and his brother sleep in bunkbeds with the "memory foam" type mattresses. About 3 times a week he wakes up in the morning to a wet bed. I've tried putting several towels under his sheets, a waterproof mattress cover, and a sleeping bag, but no matter what I do his urine soaks all the way through to his mattress.

(I don't let him drink any liquids whatsoever after 6 pm and I make him use the bathroom at least 3 times before bed.) He's just a pee-pee factory! Anyhow, I don't know exactly how to get the damp urine stains out of his mattress (it's no ordinary mattress, it's absorbent).

Does anyone have any helpful ideas at all?

Help me please! ARRRRRGH!

Jo