I think, that although he wont be fertile anymore its likely that it takes a bit longer for the hormones to work their way out of the system.
I have a rescued dog, he was neutered at the end of June. Even now he still has the odd urge, but it is getting less and less as time goes on. When he first arrived he was humping all the time (not the other dog though - nooo, my daughters coat!
), but it is getting much better. His last episode was last week, and only for a few seconds so i'm hopeful that he just doesnt feel like it anymore. Maybe its taking longer because he was used as a stud in the past, so he "knows" what to do, but either way his manly urges are lessening.
Sorry I cant help with a specfic cat scenario. All my cats are female, apart from any foster kittens, and long term foster boy kittens go to another fosterer.
Hope that helps x