My husband had a back injury years ago while at work. It was pretty serious that he couldn't go back to work. Since then he hasn't been able to reach behind to wipe his butt. I've tried telling him to try different positions, but insist that the shower was more better for him.
Other than that you actually physically helping him.
The use of damp wipes can also help. (please do not flush even the "flushable" ones) But to use them he still has to be able to reach.
There is a "wand' that can help called Comfort Reach or Long Reach they hold the piece of toilet paper and it makes it easier to reach under. Google is your friend here.