Hi CHmama,
I'm not sure why he started having issues now after 3 years. I hope someone else will know. I haven't had a CH kitten but I've seen a lot of rescues advertising them for adoption, and they are often pee pad trained. Here are some examples.
Here is another example. I'm glad we copied it because the original link to the Petfinder ad no longer goes through. I really like this idea, if you don't mind some laundry.
viewtopic.php?f=22&t=22585
Here is an adapted box I made for my cat who had trouble walking but was not CH. It's a recessed box, with the entry basically at floor level. The 8th picture down shows the finished project.
https://web.archive.org/web/20201018022 ... _litterbox
It sounds like you are wanting a box with high sides for him to lean on? They do make some low entry boxes, but I'm not sure if they're low enough, or the sides high enough. Here are examples.
viewtopic.php?p=105156#p105156
For urine everywhere, you probably already have enzyme spray like Simple Solution or Nature's Miracle. Enzyme spray doesn't just cover the smell, it actually breaks down the urine so it is gone. If you don't have a black light, that's what you need to find where to spray. Close your curtains and turn off the lights so the room is dark, then walk around shining the black light on the floor and everywhere you think might be peed on. Urine spots will glow under the black light. You'll love it!
Here are 2 examples. I don't know what's on the market now.
viewtopic.php?f=5&t=19962&p=102512&hili ... ht#p102512
viewtopic.php?f=5&t=16986&p=89066&hilit ... ght#p89066