I once read an article about “nocturnal enuresis” (aka, bedwetting) while waiting in a doctor’s office for an unrelated issue. The article addressed issues with children around 8 years of age, but I think the approach could be modified for an older child who wants to do this on his own.
The suggestion for the parents was to wake up the child at around 1:00 AM, and physically get him or her into the bathroom. The child may need some help because he/she will be disoriented. After a few weeks of doing this, the parent stops waking the child. The article said that for the most part, the child will stop bed-wetting.
An older child could set his alarm clock, and adjust the time that best works for him.
And before you dismiss the solution, this approach worked for my son, except after a few days, I personally couldn’t handle the 1:00 AM wakeup. Instead, I woke him before I went to sleep. I did this for a few weeks, but once I stopped waking him up, the problem stopped.
Wishing you Hatzlacha.