Is he constipated at all? I was told recently by a paed that when kids get a little constipated they can wet their pants because there's a full bowel resting against their bladder in a small space and they just don't have as much room in there.
Apparently another slight possibility is worms. But they also cause itchy bum and sometimes tummy cramps.
Up until a couple of weeks ago Natty was occasionally wetting too. I'm inclined to give her the benefit of the doubt - I don't think it's laziness. Just as I doubt it's laziness with your little guy, if he's sitting in uncomfy pants to the point of getting a rash, I think he might be embarrassed by it in which case I'd call it an accident.
I reckon a chat to the GP could be worthwhile?
GL.

ETA: I think N has been constipated at times so that's the possibility I'm leaning towards in our case. I'm also going to worm her just in case because although I've seen no worms (I'm never that close to her bum TBH) and there're no other symptoms, it's cheap and easy to treat for JIC. So that's what we're doing about it. (Although as I say, it has now been several weeks so I'm crossing my fingers she's over it, whatever it was.)