I doubt it's from the neuter--a year would be a long time to harbor an infection from the surgery. I'm wondering if he could have an impacted or abscessed anal gland? At any rate, it does sound like he should see a vet as soon as you can get him in. The cone is a good idea until then.
Hope all goes well at the vet! Keep us posted, pls!
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."