Hmmmm.....weak hind end, not urinating, and eating cat litter can sometimes be symptoms of acute kidney failure. Are you sure he hasn't gone pee somewhere else? I'd be taking him to emergency if he truly hasn't urinated since Sunday.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler