My 6yo son will be starting 1st grade Monday and he is just now getting to where he can wipe effectively. I saw him in the bathroom a few times trying to reach behind. He looked like he was about to fall off of the toilet, it was so awkard. So, we practiced in the bathtub during his bath. I would have him sit on his knees with them spread apart a little. Then he would reach behind until he felt his "booty hole". I explained that is where the poop came out and he had to reach that far to clean his bottom.
He has gotten better, more flexible, with age and practice.
For a while he reached from the front. Of course, with a boy there is not so much of the issue of getting the poop in the vaginal area like there is on girls. So, I let him do it that way for a while.
He has since gone back to wiping from behind. He still occasionally comes home with stripes on his underwear or dried poop on his bottom, but it is getting further in between occurances. I think it is something they just have to practice and practice at.