How can you or I tell if our bottom is clean enough?
Clean enough implies primarily that:
- The bottom will not be uncomfortable, as it sometimes is when there is some fecal matter remaining
- The bottom will not smell or leave (many) marks on the underwear
So, for the one, he has wiped enough if his bottom feels comfortable, and in general if it continues to not feel uncomfortable. If he finds that he gets itchy or feels like there's "something there" afterwards, then he knows he didn't wipe enough and can do better next time.
For the second, well, the way I do it is I look at the paper afterwards. If the paper has a certain level coloration, then I know I need to wipe again, right? I would work with him to identify that level. And, if he's unsure, give another quick wipe with one sheet of paper - if that's clean, you know you're good.
It's also probably worth discussing what kinds of stool will need more wiping (wetter stools, mostly, or the "stickier" ones in particular). Dry stools typically just need one good wipe. This could even be combined with a discussion of the science of poop - what different factors affect what characteristics the stool has.
He could even perform a science experiment on himself - measure his hydration, his fiber intake, fat intake, carb intake, etc., and see how the changes in those levels affects the stool and how much wiping is required! Very appropriate for that age, and can be fun (and if nothing else, perhaps encourage him to eat better!)