Litter box stinks out the entire unit!

We've recently adopted a year-old cat who's been very demanding, but it's totally been worth it. Unfortunately, his litter box stinks the house to high heavens. The entire place smells of litter, and if not litter, then it's pee.

I scoop out the box quite literally as soon as he does his business, or if I'm at work I'll do it as soon as I get home. There's basically not a moment that the litter box isn't clean. I've already done a full change of the litter (it's been almost 2 weeks), as well as switching his litter as well. We went from this clay pebble one that the shelter was using, but it was awful, to Catsan litter (clumping clay) which has been incredibly useful but still smelly.

I also bought him a new enclosed litter box (it's very large) that has a lid and everything. I've sniffed every square inch of the house to see if maybe he's had any accidents, and nothing! It's driving me around the bend. His litterbox is also in a corner next to a window that's always open (cat unable to get out).

The unit itself is quite small, open plan living. I'm willing to believe that it certainly doesn't help the issue but surely it all shouldn't be stinking this much, right?

Any advice would be greatly appreciated!