5 year old not wiping his butt?!?

My 5 year old is not wiping his butt..... he says he forgets, even after I remind him to........He was wiping himself for awhile then one day he started saying he "didnt know how" and would call us to the bathroom, well we never started wiping him again for fear it would become a habit. We would just remind him that he knew how and give some quick directions....problem solved, well one day I noticed an odor and the "streaks/skidmarks" started to show up in his underwear (big ones). when I asked him if he had been wiping he would say he forgot or say he didnt know or couldnt remember or just flat out say no....so lately hes given up on wiping it seems. We have given him positive reinforcement when he does wipe and have punished him when he doesnt............. Is this normal for a 5 year old? Any other parents have any experience with this? how do I get him to be a consistent wiper again?