Long post, please bear with me!
I have a 2-year old neutered boy named Kevin. For the first year of his life, his litterbox habits were immaculate, thus he grew up with free reign of the house. Even when I’d take him to my mom’s house to visit on an almost weekly basis, he’d go in the litter box I’d set up there. He just knew a litter box is where you do your business.
Several months ago, it seems like out of nowhere, he decided to start peeing right outside and up against the cage. At the time, my life was pretty stressful and I just didn’t have the time to deal re-training him, so I did the worst thing possible: I put newspaper in that spot to deal with the mess. Of course, that got him thinking that was acceptable. I wholeheartedly know that I perpetuated terrible habits from this, but am now trying to fix it.
I recently moved about a month ago to a new apartment and still kept him free-reign. Of course, I found puddles of pee all over the place. I started to keep him in his cage (this type here) to confine his space:

It’s been a couple weeks; he has absolutely no aversions to hanging out in his litter box, but he just ends up peeing through the grated floor and onto the cage floor below. I add his scent to his litter box and he does poo and pee there about 50% of the time, it’s just selective for some reason. I’m not sure if he has any health issues. He looks, behaves, and eats totally normally otherwise.
I want him to be free-reign again, he’s so bored in that cage and it makes me sad. Especially because his brother (who has perfect litter box skills) remains un-caged and he wants to be out and play with him.
Any advice is greatly appreciated!!