Ask him to change the shower head.

You can buy a shower head where you can insert another shower hose; make sure it will have water force comes from the hose.

Some residue may be still sticking on his butt, even wiping hard. I have some advice.
Make sure he poops before shower.
While showering, he hoses his butt with the shower hose, make sure the water does not go inner part of the body. It will help to clean any residue stocked in the wrinkled part of his Anus. Advise him to apply vitamin E cream or some other body cream on his मलद्वार (anus).  It will take care of itchy anus   in couple days.