our son is having a little bit of that issue too. so that last few times I've tried to go in and just observe what he's doing. Our son is attempting to wipe but doesn't always get all the "inside his bottom". with him I think it's more it's hard to reach around there. I don't know if that's the way with your son or not. so we've started reminding him when he goes in, "be sure to wipe all the way up on your bottom." And he usually does much better. Our big issue is remembering to flush the potty after an #2 event.
so maybe he's "surface wipes" and thinks that's sufficient. or he's having trouble reaching all the way around EASILY.
My mom suggested that and i was immediately, like, he's not fat, but when I thought about it the next time I went. Trying to reach behind you and perform an action "blind" isn't necessarily an easy thing to do. Maybe figure out if he's just having trouble reaching or if he's just in a hurry to get done.