People are just weird, sometimes. I worked for a family once that would still wipe the 5yo after a BM. Like, I mean, run to the bathroom to do it for him. I simply refused, 'he'll figure it out,' I said. But nope, they had to do it. Ok, whatever.

It is interesting, though, you have more confidence in the child than his own parents. We all know, the worst thing that would happen is he has an accident. In public. Where other people might see.

And I just re-read your OP, he has a big brother, so not even the first child. I guess all you can do is grin and bear it. Or how about throwing out the diapers, or sending them home? He might still come in one, but say, no more keeping them in his cubby. If they want him to wear it to Cosco, they'll have to remember to bring it themselves.