As @nikv will confirm. It takes a few months for the hormones to subside after the operation and hopefully the aggression with it.
As far as litter boxes. Just place more. It has worked for us. Multi cat household here with 8 of the little fur monsters. We got fancy schmansy cabinet ones. A couple igloos. The more boxes the better. Pain in the ass but solves the outside the box issues. Our latest rescue pees on his back paws then tracks it. Still trying to fix this issue. He uses the box. But is weird about it. He has gotten better at it tho...
Thanks for the advice! He has 3 different boxes, meanwhile my senior cat only uses one other box which he will use too. His preferences are interesting. There's one box he will NEVER poop in, only pee. That's where he used to leave the puddles.
Your cat is so cute! He looks like a lion sitting there. And so fluffy that it sticks out between his toes! Aww
Hopefully he will get into the habit.