My uncle is urinating everywhere. What do we do?

I would definitely, as said, get him checked for an UTI. This causes the person to feel they have to go all the time. UTIs are serious in older people. Is he showing signs of aggression. Is this something new.

And Grandma is correct in what she posted.

Does he have dementia? One of the things that goes with dementia are the "filters" that we live our lives with. Filters that tell us we should not do certain things, we should not say certain things, we should act in a certain way.
If he is not wearing incontinence underwear he should be.
He should be supervised at all times.
He should be on a routine bathroom schedule. Typically every 2 hours is used as a time frame. Get him up and have him go, if he says he dies not have to go, have him try anyway. If need be bring him back to the bathroom in an hour.
Absorbent pads on furniture. Waterproof mattress pads AND absorbent pads on the bed.

You will get advice, but you need to give more info to get the best help.

Does he have dementia? Will he wear depends? What are the circumstances when he does this? Does he live with you?

Also has he been checked for a UTI or other physical reason he could be incontinent?