He doesn't leak during the day or have problems during the time he is awake. Only when he is asleep. Dear trouble, He really should be evaluated by a urologist. Noctural Enuresis, or bed wetting is uncommon past the age of puberty. After a thorough history and physical, it can be determined if this is primary or secondary enuresis.
Primary enuresis is consistent with accidents throughout life, which sound like is the problem.
A work-up could be done to exclude structural, neurologic causes, systemic diseases and functional incontinence. There is no single etiology to explain enuresis in every patient. A simple physical done by the right doctor, a urologist, with a blood and urine specimen sent for analysis and culture could answer some basic questions. There are also medications that can be taken that when given at night may limit the amount of urine made. I m sure you understand that the more fluids you drink in the evening before going to bed will increase the amount of urine that needs to be eliminated a night.