How do we stop dad from pulling his depends/diaper off since we can’t get pants on him?

Also we use disposable bed mats with washable bed mats under them. The mattress has a cover on it too. For the most part, disposable mat gets it all and you just toss it. If it gets past the disposable, the washable will get it, and you simply wash it. Nothing has ever reached the mattress cover yet here.
So again, we use hospital gown, disposable mats, washable mats, and mattress cover.
Out of sight, out of mind when it comes to them removing diapers.
Hope this helps.

We use a hospital gown at night for our dementia loved one. He can't figure how to take it off since the ties are in the back. Also since the diaper is not visible, he doesn't know it's there.

Could you try a sarong tied around his waist? In bed, it feels a lot like loose pajama trousers, but it can be opened to get access to the depends.