G.W.
My son is 5 and if need be I still help him wipe. It's such a tricky thing to learn to do well. But if you're son is simply NOT wiping at all, you could try some tough love, but it usually works real quick! Explain to him that if his bottom is not wiped and clean after using the potty, you will need to give him a quick clean in the tub. Tell him it wont be a fun bath and he probably wont like it. Most likely you'll have to follow through-- The next time he does it, stand him in the tub and rinse off only his bottom half from waist down with cool water. (unpleasant) after a couple times of that I bet he starts wiping to avoid it!!!! And it's not done in a mean way or anything, it just "has to be done quick if you have a dirty bottom" Can also try making him wash his own dirty underpants if that would bother him enough to quit.
Good Luck!!!