Suddenly not using the litterbox
Well my kitten is about 8-9 months, he was a rescue so I dont know his exact age. I found him with his sister when he was about 4-8 weeks old. He was neutered at 5 months old. He's never missed his litter box, though the day before I got him neutered he did spray once on my chair. it was immediatly cleaned up and he hasnt since. Over the past 2.5 months I have been using a toilet seat insert and they've been doing fine. For the 7 weeks the insert has been in the toilet with absolutely no problem. My other kitten uses it 100% as does flash, the cat in question. The problem started last week. Flash suddenly stopped using the litterbox. There was no change in litter or change in food, but he started showing some 'sexual' behaviors as well, kneeding my dog as if he were getting ready to mate. He has been urinating and pooping around the toilet purposly, not missing. The other kitten still uses the insert but he suddenly refuses. He's urinated on the dog bed as well as infront of my reptile cages. All these behaviors started one week ago, and nothing has changed. It leaves me stumped, but I cant have him continue to do this. Its come down to me locking both kittens in the large bathroom when I am gone or at night.
What could cause this and what can I do?