No one has any ideas?
To add to this, funnily enough the day after I posted this he's started peeing next to the litter boxes again. He doesn't even try to hide it, he'll do it when I'm right around the corner, and in a way he actually announces it with a frustrated, loud and deep MEOW (after all this time I've somewhat come to understand his different meows).

I'm just so lost and don't know what else to do...