Well, I don't have any suggestions on how to stop it. What I can tell you from my personal experience, is that it sounds like he's not ready. When my oldest son potty trained, it was completely. He didn't need anything at night. My youngest, however, is 6 yrs old and still wears a diaper or pull-up at night. He may go several nights without wetting, but the night I don't put something on him, he wets the bed. He does not have accidents during the day. I work with a woman whose son is 13 and still has accidents. Boys bladders mature at different times. Unfortunately, there is nothing that can be done. They wet when they cannot consciously stop. My Thomas, when he wets at night, it actually happens just before he wakes. I do have a friend who "layers". Mattress cover, sheets, mattress cover, sheets. So when her son wets, she just takes the top layer off, rather that changing the sheets completely. I have spoken with his doctor and have been told not to be concerned. If it's a temporary thing, you might think UTI, but in our case, it's not a temporary. It is getting much better, I'm just a little gun-shy to go without anything yet.
Good luck!!!
K.