Did you change the litter brand by any chance? I know years back when I changed litter brands my younger cat refused to use the new litter. As soon as I switched back to the old one the issues ended.
I also agree that putting a box back where it used to be would be worth a try, and a second litter box also. Many cats don't like to share and yours maybe one of them. Good luck and thank you for taking your kitty in to be neutered, it may not help this issue, but it won't hurt either!