Advice on the last 5% of Potty training. WIPING

My 4 year old has been been deemed potty trained for 8~ months and Is actually pretty darn reliable. Almost never makes an oopsy, Never wets the bed. Will go all on his own.

But the concept of effectively wiping continues to allude him. Every time, he will go number 2 and makes a half-hearted effort and then calls for us to come help him despite us explaining and demonstrating every single time. I don't know if it's because he feels like his little arms can't reach or hes got tight little butt cheeks, but he can't figure it out. I will hold his hand and walk him through the motions and show him, "Look, Because we went all the way down we got more poop, we need to keep wiping till it's all gone" But he gets frustrated and gives up and won't commit. He just brushes the outside.

Anyone else deal with this? Any tips to help them understand wiping better? I never had this issue with my oldest. She figured it out in less then a week.