My only suggestions are:
--keep putting the cat into the litter box everytime he pees on the floor. Take his paw, gently, and rub the pee and then the sand with it too.

Also, put the litter box in a private place. No cats or humans that I've ever seen, like to be watched. Nevermind humans and cats, my turtle doesn't even like to be watched. I have Zezu's litter box in a small closet with the door open just enough for him to fit in. His own private bathroom. I've walked in on him a few times by accident, and he gives this shameful look, like he's saying "Hey, come on, get outta hear. hello! someone's in here, wait your turn." I just slowly back out as not to spook him. He usually bolts out asap.

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