Cat weeing outside box

[Yumdoritos]

I know this will have been posted a hundred times before but I've tried everything I know and haven't managed to help, so maybe I'm missing something

My partner in Germany adopted a cat (I'm in UK), I went with him to get it from a shelter about a month and a half ago while I was visiting for 2 weeks. Cat is an indoor cat, 8 years old and has a leg issue, loses balance and falls a lot or puts weight on his legs wrong - stands on hocks occasionally and puts weight on wrists sometimes when sitting, can't jump higher than a foot or so.

Cat was behaviourly fine when partner first got him, used litter tray regularly. Legs have already gotten stronger but he isn't very active. Isn't standing in weird postures so much anymore either. I have 3 cats at home but partner hasn't had one before.

Cat did a wee on partners bed, turns out the litter box was full and stinky, cleaned it up and changed litter, cat went back to normal routine using the box - I was at home when this happened. I went back 2 weeks ago for 2 weeks (got back home yesterday). First few days I was there the cat was using the box. Always cleaned and not stinky.

We went to a pet store to buy bulk litter and got some wood litter to absorb the wee more as it was just sitting at the bottom of the tray before. Mixed it with old litter but cat wouldn't use it, did a wee and poo on the floor in the open. Emptied the box and got some clumping litter the same brand as the original litter - looks and smells identical. Cat wouldn't use it and did a wee inside the cat stand. Went back to original litter and cat still won't use it. Cat has been holding it in all day and at night will go and wee and poo in the room somewhere, after searching around for a place, so it's not like he can't hold it in because he does. Put him and the litter box with fresh litter in the bathroom when we saw him searching for a place to wee. Did a wee on the floor in the bathroom still rather than use the box.

Got feliway, only been a couple of days so can't tell if working for sure yet but cat is still having accidents and hasn't used the litter box once at all since swapping from original litter, even though that litter is back now. He's had a vet check as he had to go back to the shelter for a booster jab and was checked then. Had blood test for diabetes too and doesn't have it. Doesn't eat or drink excessively either. Ordered a new litter box now as maybe he's scared of the old one for some reason?

Seems to have settled in OK in general though, friendly and purrs and wants attention and cuddles with you, lovely cat. Any suggestions what else we can try to make him wee in the litter box again? Lots of info from me here but wanted to explain the full picture!

[Yumdoritos]

Also the layout of the room or furniture etc hasn't changed from when he used the box. Thought it might be my presence there as he seems to prefer me to partner, but when I was there and now I'm gone he has weed outside the box again. Keeps getting covered in it too as he walks low to the floor and falls with his key issue so he's had to have a mini bath tonight :(