Bathroom etiquette

So I really didn't mean to have my mind in the toilet today, but it just seems like the topic won't go away! I took the Monkey Man over to my friend Amy's house for our weekly science co-op. As we were about to load up in the car to go home, I am informed by the Monkey Man that he needs to poop. Such impeccable timing! So we went back in and I helped Amy clean up a little more while the boy did his thing. He finishes, we say our good-byes and out the door we go. Well about 6 blocks after we left I thought to ask him if he had flushed. To which I was told, much to my great horror, that he had not! Seriously!?!?! I am at my wits end here! For months I have gently reminded him to flush, I have yelled at him to flush, I have threatened to take toys away if he doesn't flush, but NOTHING seems to make an impact! So, at this moment, his is in the kitchen writing, "I will flush when I poop." twenty-six times. Then I'm going to make him write an apology to Amy for dropping a deuce in her crapper and not flushing! I am wondering if this will have any impact whatsoever on his actions. I guess only time will tell. But I swear, if I open up the toilet one more time to find an unflushed present, I may have to give him a swirly!