I got tired of "I'm done" and started telling him, you're a big boy now, big boys wipe their own butt. I went, taught, watched, checked, redid if I had too, checked his hands, made sure he washed a dried. He still ask from time to time, usually when it hurt him from holding it in so long, but even then, I just tell him, you know how. I have flushable wet wipes on standby for him, but he never uses them. I occassionally find poo on his undies but that's to be expected and he will learn better as he gets older. My son is 5 now. I started this about a year or two ago. If he refuses, maybe you can make it worthwhile by offering a prize of some sort for wiping. Give him a nice snack/treat. Then extend the time between. Give prizes after a week of accomplishments, then 2 weeks, and so on and so forth.
Good luck and keep us posted on progress. :0)