"Urine Off" definitely works wonders (we sell it at the clinic I worked at before I had kidlets), BUT there is a much cheaper alternative The way Urine Off works is that it breaks down the proteins in the urine that make the smell. Something else that does EXACTLY the same thing is an enzyme-based laundry powder like BioZet. If you are trying to get urine smell out of carpet you can mix the BioZet powder with a little water to make a sort of slurry, and work it into the carpet, then rinse it out. If you are cleaning walls, then you can mix it into water in a bucket and just wipe the walls clean. You can also use it easily on outdoor areas by making it up in a spray bottle and spraying it on anything the cat urinates on.

Obviously desexing would be tricky in your situation because the cat is probably "technically" still owned by someone else. If you find out you can take legal ownership of the cat, then I'd get him desexed ASAP. Your vet is right that it may not stop the spraying though. It depends on if the cat is doing it for hormonal reasons, or if it's now behavioural. If it's hormonal, then desexing will help dramatically. If it's behavioural then things might be a little trickier.

I'd also agree with krysalyss that getting some Feliway is a great idea. It's like one of those room aroma things that you plug into a wall socket. The pheromones that are released are "happy kitty" hormones, lol!

Good luck! Tom cat urine smell is one of the worst smells ever!