You can take him to the Doc, just to rule out anything.
My daughter's Teachers, from Preschool to Kindergarten to 1st grade, ALL said unanimously, that kids these ages, have accidents. And yes, even at school. And it is normal. Except, that the parents are embarrassed about it but ask the Teachers about it, in a hush-hush manner. And, at my kids' school, the Health Room even stocks... clean underwear and shorts and t-shirts, for the young kids that do have accidents at school. And they make NO fuss about it. Because, accidents, happen.
You see, I know this because my daughter had accidents in Kinder and 1st grade. And in Preschool. And she was my first child. I wasn't sure, what was "normal" or not, back then.
She is 10 now and perfectly normal.
Have you tried, to chat with your son, about it? In a non-lecturing manner and in a manner so that he is not afraid to express himself about it? See what he says. Or, maybe him being in Elementary school now, he is getting adjusted and so many things can tweak a kid. Maybe, he is holding it ALL day... and by the time he comes home, he is bursting...and then it just comes out before he can get to the toilet?
Some kids, DO hold it, ALL day long while at school. They are self-conscious to poop at school. Maybe that is the case?
Ask him, in a comforting manner, what may be bothering him.
And I agree with your Husband. Try not to get "mad" at him. I highly doubt he is doing this on purpose.