Yes, Pentan. Neutering will stop a tom cat spraying, though I'm not sure how long it takes. I suppose the hormones need to work out of his system. It worked for my ex-feral cat, though he was so wary it me three years to get him into the house at all (when he was whisked off to the vet's for his little operation) and then another year before he moved in properly. Now, after a few years in the house he doesn't bother to go out much at all and loves to sit on my lap or "help" at the computer.
Good luck with the cat, it's good to hear he's found a potential home. Let us know how you and he get on.