When will toddler stop pooping at night?

My 18-month old has pooped almost every night for the last several months. Just recently it's led to horrific diaper rash that we have a hard time getting rid of. He also poops during the day occasionally but rarely does this lead to not having night poop. Even on the days I wake him up he already has poop---sometimes I think he poops right before bed but usually I think it's very early in the morning and he goes back to sleep. Maybe I should feel lucky that he sleeps so soundly--poop and all--but I'd rather he have a healthy bum instead! Has anyone else had a toddler that did this and did they eventually grow out of it? I have asked all my friends and none of their kids poop at night. His pediatrician isn't concerned.