Hi
@Dianne, I am sorry that your husband is going through this and that you have this extra worry.
I don't know how common it is, but it is definitely something that happened to my husband too.
It seemed to be triggered by fear of falling as he was unable to understand how far he needed to bend in order to sit down.
A raised toilet seat (red) helped for a while - but eventually even that was no real help.
Like your husband, my husband would fight against all coaxing to get him seated on the toilet, his legs would stiffen and his whole body would become rigid and he would cry out in fear. I tried him with a commode, but he fought against that as well.
Even if I did manage to seat him, he would be fighting to get off the commode/toilet within seconds.
Fairly soon after this, he began to lose control of his bowel motions and so we moved him to pull-ups which removed him from a very stressful situation. It helped that, by this stage, he had lost any self-consciousness about using pull-ups.
Having said that, even in pull-ups, he was still happy to use a portable urinal when it came to having a wee, as it didn't involve him sitting on a toilet.
We maintained this situation for a few months but eventually he became doubly incontinent as his condition worsened.