B.,
Try cutting them off of wheat and sugar. My husband, who is 27, has the same problem, it's called primary nocturnal enuresis, and this worked for him. See if they have any food allergies or any allergies of any kind as these can aggravate the problem as can illnesses.
Nocturnal enuresis a chronic bed-wetting disorder that can last years and it's quite common.
This is the website for a clinic that can help if nothing else does: http://www.drybed.com/index.html
They have had amazing success with treatment of people of all ages. If changing his diet didn't do the trick, we were going to contact them next.
I have not heard good things about limiting fluids or bed alarms.
It has also been found to be something passed genetically, so I would check in the family if anyone else has ever had this problem.
My son is only 3 and so I have yet to discover if this is an issue for him as well, but I am definitely going to keep an eye out.
Good luck with everything.