M.R.
I wet the bed until I was almost 12 years old. I slept like the dead, would rarely wake up to use the bathroom, and it didn't matter how much or little water I drank. Keep something under his sheets to guard his mattress and if he doesn't mind the under jammies, use them. Keep in good touch with his pediatrician, but this is developmentally normal in a lot of cases, even if it is not ideal. If his attitude about it is good, just try to be relaxed. My younger sister and I went through this and it was soooooooo embarrassing. I'm very thankful that people know now how common it is. I would also probably not even try to the alarm thing, especially if it doesn't work for him. He might just need a few years to finish growing into himself. That's awesome that you're taking such good care of him!