You ask a question I can not answer as to another. Each has the same problem, but the underlying reasons may differ. To know the reasons answers the question of how to teach to it. For me, I honestly did not know when I had to go. I could not keep track of it and my other senses too. To pee was a lifeline for me. To produce something told me I existed. But, I think another autist might look at it the same way I viewed sharing ideas, as if they were giving up a piece of themselves to do it. Poop is especially like that I think. Like everything it is individual in odor. You can mark out a world for yourself in poop. We are territorial animals at our core. Space is a big deal for most autists. I used my urine, not feces, to mark my life. It soothed me, my own smells. I wet my bed until late into my teens (not what you want to hear I’m sure.) I was day continent late too, by age 10 or so. But for me it was more an issue of my using it than training it. One of the first lessons I learned was that pee could rid you of people's expectations. The program they used to teach toileting sat you on the potty until time ran out or you peed. Then they worked you; how long I don’t know, but I knew it was for a set time. I was task avoidant, Mom calls it. I learned quick to wait, then make myself pee at the end of the time for sitting. Time for work was instead used to clean me up then. They fed us lots of juice so making myself go was easy.
Peeing is a pipe in your stomach that tickles. To make myself go I don’t feel the tickle. But to have my body declare it sets me off in to uncontrollable giddiness. Happy I was not. I pee a lot of my own making to avoid the body overload. Mom says I ask all the time to go to the bathroom. I need to go, but not for the reasons you go. I go to stay empty. You go when you feel full.
Poop is harder. I used it to scare off workers some. I used it to play when bored. I used it to stim. I am not alone in playing in poop I know, but it was a childish choice that made me look very bad. Not everyone’s problem is a bad choice though. Many don’t get the connection of putting it in a bowl of water. Some may find the flushing aversive. Sitting on the toilet is without ground; to sit backwards and hold on to the seat back would be better. My cousin learned with a musical seat. I would have liked that one.
Poop is crap to you, but it may mean something else to him. It is the best answer I can give you.