Aw sounds like he's quite happy in there. It also sounds like he's using the litter tray as a sand bath rather than a potty. I always had to move the potty to the chosen pee area with Charlie! Newt used it where I put it but maybe that's just because it was the room next to his nest
Although recently he's been hoarding in it and I suspect peeing under the house! (Since it was put on legs).