sadly, i dont know that there is a fix for this.
i had a cat for 3 years that sprayed every year in the spring. sadly, a week ago monday, he disappeared (because of the spraying we didnt allow him in the house, but did keep track of him outdoors - he stayed with our outdoor dog - we live in the country)....
anyhow, i had supposedly had him neutered, and there was an undecended testicle, and they supposedly went in (like a spay) and looked for it. now, the vet never told me if they found anything.. so... i was suspicious when he continued to spray..
finally earlier this spring i had taken him in to figure out what was going on. this new vet looked at him and was pretty sure he was not completely neutered. she could also smell it on his fur. they took blood and gave him something and took blood again and sent the samples in to be tested for testosterone. he tested positive, so they wanted to go back in again and find the tissue that was there and remove it.
however, she did tell me that even if they did, because he was older when he was neutered, there was a chance he would never stop spraying.
i am also told that they can be jealous of both other pets in general, and kids.... and you said he was spraying before he was neutered... so.. you might have the same issue as me.
it may cost a lot, (it cost me 80$) but maybe having him checked for testosterone would be worth it. however, like my vet told me, there is a chance that he will never stop spraying. there are sprays you can buy to discourage spraying but i dont know if they are safe indoors with kids.. we used some outdoors, and it helped, but we cant afford to keep buying a can of something - every time it rained it just got washed away.
im sorry to say that he may never stop... :( and a spraying cat is the NUMBER ONE reason cats get killed, put in shelters, and 'set free' outdoors. we had our cat for 3 years here, and he never once disappeared, so of course we think he is gone for good. he was here every day, and always waited for his food in the morning... and always came when he was called ! :(
we are currently in the process of looking for a female cat to keep indoors exclusivly. after all, females dont spray. this is one of the reasons that cats should always be immediatly neutered as soon as they are old enough. spread the word in your community so that someone else doesnt have to go through what you are going through.
im so sorry that there is really no stop to this :(