He will eventually grow out of it. There really is nothing you can do. You can wake him up, etc., etc., but he will still continue to do that. You can withhold drink (thirst him to death), but he will still do that. He will just have to grow out of it. Heavy sleepers do that. I have a friend who did everything and I mean everything to get her firstborn to stop wetting the bed...he had to wear 'diapers' until age 11.
I understand that's normal. I've been lucky because all of my children wake up with a dry diaper at age 18 months. They drink til they go to bed, take a sippy cup to bed and they don't pee in the bed, as long as they go potty before they go to bed.
Just let him know that it's not his fault, but he will grow out of it eventually. Hopefully sooner rather than later.