My son did this - I got the normal developmental speech from the doctor. We got him up at night, but no one was getting enough sleep. When he got older, this problem really had an impact on his social life - no camp, no sleepovers, etc. The pediatrician referred us to a pediatric urologist who explain there's a condition called nocturnal enuresis which basically means not waking up at night to pee. He told us about a well-tested medication with no side effects (rare, I know) and that he has teens and even a few college freshmen on it. I'm not a big believer in medication when other things will do, but I wanted my son to have a normal life. He went on the meds, did great, and started going to sleepovers. When he went to another house, I sent 1 pill in a blank container and told the other parents to give it to him at night, that it was for allergies. Worked great. Same thing happened at sleepaway camp. He went off on his own at 11, the problem came back, and he went on again until about 12. No problems.
While your son might not be there yet, I want you to know there is a possible answer for him. I also hope this will help those people who are reading this post who have older kids.