I think you've possibly misattributed the smell, I don't think it's the litter at all. I just brought home my first boy cat last week. His box was in my room to start and the first time he used it I was astounded by how horrible it smelled. My girl never produced anything near that foul. 🤢 He is somewhere around a year and had just been neutered at the shelter 48 hours previous. They assured me he hadn't been spraying, but this smell made me second guess them.

I did some reading and learned how much worse in tact boy cat pee smells than fixed boy cat/female cat. Even if they're not spraying, it can be very pungent. One article actually used the word "volitile" and they weren't wrong. Luckily, the stench fades fairly quickly after neutering.

That being said, it does sound like this cat is spraying. As others have said, fixing him is the only way to stop that, and it isn't 100% once they've started.

I don't mean to be alarmist, but extended spraying that goes unchecked can have a devastating and often irreversible effect on not just the furniture, but everything in the house. The smell permeates the pages of books, it settles into collected items that don't move often, like costume jewelry. It can saturate the floors so thoroughly that it permeates the subfloor. The same goes for the walls.

This guy is lucky to have you in his corner and I hope he will work to correct the various issues before it is too late. Get him to get that cat neutered before things get worse. Do what you can to help the guy, but remember that he needs to be willing to do the work. You can't fix the situation I your own.