Believe it or not, my husband went through this for years while he was growing up, and also my step-son at times, who is now 12 1/2. The problem was more that they were sleeping too hard, than an actual bladder problem. This may be a little difficult at first, but my mother-in-law had to wake him (he will only be in a semi-awake state if this is the case) and walk him to the restroom right before she went to bed (since it was later than when he went to bed) and once during the night, etc. If he is in fact, a heavy sleeper, he will return to sleep with no knowledge of the event. Make sure the doors are locked to the outside, for, as he gets used to his body getting up to go at night, he may walk toward the door unknowingly. His body will begin to walk himself to the bathroom at night, in time, out of muscle memory. Also, there are waterproof "sheets" that can go on the bed underneath normal sheets to save your mattress. I am sure you are stopping liquids at a certain time of night (say 2 hrs before bed to start out with), and make sure he uses the restroom or at least tries, every 30 minutes before bed, and once immediately before.
Hope it helps some.