It may be maturation, but also keep an eye on what is different about the nights he does have accidents. His may be reacting to something in his food/environment causing him to pee in his sleep. Maybe he had Orange Juice with dinner, milk that evening, ketchup on his fries --many people have different triggers, so stay aware of what may be influencing his patterns.
Some kids also have a lot of trouble with enuresis when they are overly tired and fall into a deep sleep. When my daughter is in this situation, I'll put her on the potty about 3 hours after she goes to sleep and talk her through her potty trip, she pees, but never totally wakes up.