Strong smell of ammonia throughout the house

[CeliaFate]

Always have a litter tray available in the house for our cat, who's FIV+. He's 12.
Yesterday there was a really strong smell of ammonia throughout the house.

I changed the litter tray, cleaned all food and water bowls but it's right through the house and it stinks.
Googling suggests it's a kidney problem so I'll have to take him to the vets, but just wanted to ask here for any advice on a) how to help him in the meantime, and b) how the hell to get rid of the smell?
It doesn't smell like cat's wee, it smells like ammonia.

My old mog had a kidney infection years ago which led to him peeing in a few spots.... I didn't notice until the smell emerged as he was relatively secretive about it. I did solve the problem by watching I'm closely and sure enough, he tended to go back to the offending ares to repeat the process.... Ended up narrowing it down to 3 patches. Washing the areas with bio washing powder will help but it has to be bio to breakdown the enzymes in the wee. If you use anything simply scented like carpet cleaner, all the cat will do is think "why is this silly human covering up my important wee smell" and do it again. Tl foil over the areas can also deter them from going there again as generally speaking cats don't like to tread on foil. Feliway spray can also help. Yes for the vets, the sooner the problem is gone, the sooner the wee will hopefully stop!