My step dad never wore a seatbelt. His first wife was killed in a car wreck. The seat belt cut the artery in her neck and she bled to death.
My first stepfather's father owned a towing company on the CA coast. My stepdad saw so much nastiness in his teen years from cleaning up those messes that he installed belts in our classic car that was grandfathered in to not need belts.
Your stepdad needs to take a step back. He was part of an exception. Far far more people get killed because they are NOT wearing proper belts than those who were part of exceptions.
I had friends in grad school who had a baby. They were taking a long trip in their van, and the baby needed to nurse. The dad, for whatever reason, refused to stop the car, so the mom got the baby out of the carseat and nursed the baby on her lap in the front seat. Just at that time, they got into an accident. The carseat went out the window and landed in a way that would have probably killed a baby sitting in it.
They took it to mean carseats are useless.
But the reality was...the carseat wasn't installed correctly. And with the baby's weight in it, they can't say that the seat would have landed like that.
And without the last 15 minutes of hungry baby screaming and noticing your wife crawling back to get the baby, would he have been more aware of the road and conditions? If they'd pulled over, they would have missed whatever caused the accident entirely.
Their emotions of being in an exception have taken over; and the same happened with your stepdad.
A properly fitting seatbelt wouldn't have done that. That is where he should have directed all of this; at the industry that uses a 5'7" female/5'11" male as the way to fit a belt, when many people don't fit those ranges. Thankfully now you can get things to help the belt fit. Thankfully now there are many cars that allow you to adjust the seat WAY up so the car fits you better.
He should focus on those new things, rather than the absolute fluke of an accident combined with a poorly fitting seatbelt.
Sorry for your step father but why is it that whenever a seatbelt issue comes up someone crawls out of the woodwork with an " I knew someone who was killed by the seatbelt" story? Statistically that is not the case.
Because when you're part of an exception, it's emotional. It can feel like a sign. And I don't think the OP brought it up to say "don't wear a seatbelt", but rather just to show the feeling behind such a decision.
OP, I'd tell the cop that it's illegal, and if he doesn't do anything I'd take it higher. If I were in such a situation, I might let the cop know that I'd be happy to testify in court that he knew of the illegal situation and let it continue, if something happened to those children.