There's no way he could sit on that toilet like you do when you take a crap...
What actually happens is, his shit doesn't just go its own way after he's pushed it out. It just forms a rocky extension, like growing a horn, except from his ass. So what he does is, he "shits", making a horn come out his ass, then he lowers his ass into that grinder to grind it down. There's no discernible asshole on him any more... Just a smooth butt, because you can grind down the crack.
Or it's an eastern-style toilet, and he perches with his feet on those bits that look like handrests. That would explain the shit being all over the place, as well.