My father is 99 and has started having really messy bowel movements. Any suggestions on how to manage?

I would get him checked by his doctor to rule out any medical cause.

Is it possible for someone to help him in the bathroom to find out what is actually happening? That would be my 1st step after scheduling an appointment to rule out any medical cause.

Add Sweet potato and white rice To his daily diet

Definitely a diaper would be a good idea. An actual adult brief diaper with tabs. Not a pull-up because one of those will make an even bigger mess when you're trying to change it.

Citrucel might work. People think it is for constipation, but it works well for diarrhea and honestly regulates the stool with a bit of good roughage so there is bulk to the stool and it doesn't go all pasty. Be certain he is not on stool softners. Don't use metamucil as it ferments in the bowel in a way to give many seniors increased gas and bloating, which can lead to explosive stools. If this works and he can tolerate it that's great. Don't give quite as much liquid with it, so that it forms the stool a bit more. If you haven't changed diet you may consider asking for a stool exam. He hasn't been on antibiotics? Generally people tell you to use probiotics, but actually new studies show they do nothing much at all to help. Good luck.

Have you tried imodium, or any of the other capsules known in my house as 'cloggo'? Has he stopped another drug (eg an opioid) that had a binding effect? My doctor once said that he had patients who had been taking imodium for years with no apparent side effects, and just as many patients who were continually dealing with constipation. I've recently worked out that too much liver pate sends me running first for the toilet, next for cloggo. It's the first option to try.