My son poops all over the toilet SMH

He is 8. Its my stepson, I've had him almost 4 years. Over the last year he started not wiping his butt well, or at all, I don't know. He SWEARS he wipes, but the proof states otherwise. The smears have extended from just his underwear to regularly being smeared all over the back of the toilet seat, UGH! Then he leaves it! We've talked about it nicely. Bought baby wipes and Clorox wipes to help him make it easier to clean up. NOPE. It's been about a year of him leaving a trail. I have gotten angry. I have explained WHY he can't leave poo ANYWHERE, unhealthy and disgusting. We tried rewards and punishments. I just got up to pee, in the dark, for likely the 3rd time tonight, and happen to hit the light switch this time, AND THERE IS POOP ALL OVER THE SEAT AGAIN!!! I'm gagging realizing I sat in that...AGAIN! I'm so grossed out and frustrated and out of ideas. 💩😖