I also suggest staying in there with him. I had to do that with my son for a while. He loved shoving things down the sink.

Oh honey have we been there! My daughter was a painter as well. Such a horrible phase of life - felt like we couldn't leave her with anyone because of it. Luckily she outgrew it eventually - only because she became intensely constipated and went through another horrible BM phase - of not having any and needing medical attention for it. We basically tried staying on top of her and catching her ASAP when she went so we could clean her up before she could get to painting. I wish I could be more helpful but all I can do is send you a cyber hug!

My daughter did this exact same thing on and off for about 6 months and then stopped. We went through a lot of Clorox wipes. I sat in there with her with a book. She would scream bye mommy and take 40 min to an hour everyday. I think it started because she was constipated and was trying to get the stuff out. Then to get it off her finger she would wipe it on the counters. Then it became a habit. We put a few drops of olive oil in her pasta and feed her strawberries everyday all year. This makes it easier for her to go. Just be patient and get through this phase. Soon you will be worrying about something else.

Suggest staying with him when on the potty - every time he goes.
A social story may help to learn how to potty properly without all the mess - read it a LOT when on and not on the potty - have many pictures in the story so he also visually understands. Suggest to give him something special he likes to hold when on the potty - if his hands are busy, and he is listening to a potty social story and looking at the pictures in the story he may not have time to make a mess. Suggest to have him SIT on the potty - no standding yet - too soon for that if he makes such a mess.

My daughter is 6 and still in pull ups, and if we do not get to her soon after she has pooped she will take it out of her pants and smear it everywhere. Sher has sensory issues and the smell and the tecture of it does something for her. It is a nighmare for us but she likes it. We have gotten it under control where she does not do it that much, but she still does if we do not stay on top of things. Unfortunately for now you may have to follow him in the Potty and make sure he does the right think, and compliment him when he does.