Litter box question

We have a senior indoor cat who recently has been using his litter box with his rear just making it inside the box. (It’s a large, covered box with plenty of room to move around). When he does this a good portion of his urine ends up on the floor outside the box. He isn’t spraying; he just stands in such a way it happens. It’s becoming more and more frequent and I’d love a suggestion if anyone has experienced this.