Hi, I was wondering if there are any good tips for getting my bunny re-litter box trained?

When I first got him (5months), literally the second day my bunny had already formed place where he would like to pee and poo in his cage so we upgraded and got him to the litter box. He used that too, no mess at all. About a month later we got him neutered and his clean habits stopped. He liked to poo everywhere! And he would even move his liter box and pee in the same corner.  Not sure why, haha. I didn’t change the litter box or type of litter that would provoke to him stop using it, he just didn’t want to anymore. So just recently, I bought a bigger litter box, and the scattered poo has seemed to die down a lot, and I locked the litter box to the cage so he can’t move it and pee on the floor in the same corner again. I’m not sure what is wrong? I want to get a new bunny, but I can’t if he doesn’t learn to poo in just his litter box. I’ve tried using the hay trick, but since the type of litter box I have doesn’t really allow me to put hay in the box, I put it next to it, but that seems to make him poo everywhere even more.