Please give me suggestions for cat litter boxes or how to teach my cat to pee INSIDE of the box. My cat used to do all of his business inside of the box just fine, but recently he's started walking in, not turning around, and peeing in a way that makes the pee go out of the cat flap and onto the floor outside. I've shoved his nose into it a couple of times to try to teach him that it's bad but I don't think he's getting the message. He's a pretty big guy so I thought maybe he just didn't feel like he had enough room, but his sister is basically the same size and she doesn't have this problem. Also, I haven't found a box that's noticeably bigger. They each have their own identical litter box so it's not an issue of them not having enough sand or poop area to go around. Please help, this is getting ridiculous.

picrel it's not the exact box I currently have but it's pretty close