I have 2 sons & I would be mortified for them to do such a thing as personal as that. My mother-in-law has 4 daughters & i son. Out of the 5 adult kids only 2 have ever helped out. One being my husband, the other his sister who treated her mom very roughly & to the point of me calling elder affairs who gave her a very stern talk. She would make coffee in the morning for her mom & change her diaper & leave with her mom's debit card & not come back until well after midnight leaving all care to her brother. I was so appalled when he told me & I said I felt that was not right by anyone's standards. SHAME on those 4 very spoiled, self-serving & self-centered so called daughter's.
Your husband will feel uncomfortable at first but he'll get used to it. I'll never forget the first time I helped my dad to the bathroom. I was afraid he'd be embarrassed. He took it in stride, so I was able to do the same thing. It's really no different than feeding them.
I interviewed several men in my book and a couple were in the stage where they changed their mother's diapers. They just got to where it was nothing. Just something they did.
Sexual tabus fall away in caregiving (unless, of course, there is abuse, but that is a different topic). In your case, please encourage your husband to look at this as one more thing he can do to help his mother stay comfortable.
Take care,
Carol