S.H.
Each child is different. In time, he will get better at it.
It's normal and not to worry. It's just their fine motor skills, and granted, it's not easy reaching back there and being all perfect about it.
Maybe, teach him that he can either sit OR stand-up in order to wipe...for some, it's easier doing it while standing.
My girl, when she was 5 years old, still needed help. She just turned 6.. and only now is she doing it herself, and she can do it well now, without skid-marks left.
Don't worry.
All the best,
Susan