Hi Kez,
My eldest son had this problem when he was about seven. He was given a mild sedetive medicine and a star chart. Although your son is too old for a star chart the sedative may help. I was told because my son was a light sleeper his body didnt shut down properly and some parts, like his bladder, was still 'awake'. After three weeks on the medicine and doing really well with his star chart he was dry, he did have the odd accident but nowhere like what he had been. I was also told it was hereditry, his dad had the same problem but his stayed with him to adult hood as his mum was too embarrassed to seek help.
Take care,
Carol x