How can I help my husband to wipe his butt instead of showering it?

Bidet toilet seat might be just the thing for him. There are some you can just put on your current toilet, no need for a new toilet. You do need power near by.
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.

My Son- in Law suffers from Hemroids and installed a device on his toilet that acts like a bidet. Cheaper than a separate unit but you may want to have someone handy install it for you!

Have you considered installing a bidet?