I ,wonder about that too... I noticed that since using CPAP, I no longer need to get up in the middle of the night to void. I read an article that said low levels of oxygen can keep you from producing the antidiuretic hormone... so I wonder if bed-wetting in kids could be related to sleep apnea.
My kids had night terrors about that age, as did other kids in our neighborhood. We found that it was worse when they had ice cream before bedtime (dairy?). When we stopped giving them ice cream after dinnertime, the terrors stopped.
Or perhaps the experience of having surgery was upsetting, and he still hasn't recovered emotionally?