I don't know the way to bring it up because I don't know them. One option might be to mention that lots of children that age don't know how to do it properly and so it's good if they are teaching him to do it right, because then at school he'll be able to do it better than almost anyone. I've known adults who clearly don't know how to do it right. And it's because I've NOTICED the problem. Others maybe did it wrong but it wasn't bad enough for me to notice. Some people simply never learned. Just like probably over 90% of the people don't know how to clean a toilet correctly but many think mistakenly that they do.
However, the sad news is that they probably not only aren't teaching him right, but also they probably don't do it well themselves. In fact, maybe if they were doing it right, he'd be able to by now. Basically, you need to run the toilet paper, fold it some, run it again harder, and no one can tell better than the person who needs the wipe how much help it needs.
I myself sometimes dampen a papertowel a bit and use it, then dry with a dry portion of the papertowel, because that gets the job done better probably than well over ninety percent of people. The disadvantage is that the papertowel needs to be discarded in regular trash so it's better to use toilet paper for the first pass or two.
So, it's something that to do extremely well isn't as easy as most people think. But at the same time, they should be teaching him and they're probably not.