First of all, thank you and your son for taking this baby in to your warm home. Next, I guess if you suspect it belongs to your neighbor, you have to at least ask them about it. Maybe they really don't want him anymore and if that is the case, I'm sure your veterinarian will work with you. Our recent adoption, Butter, had to have belonged to someone at some point, but I'm convinced he had been on his own for a while. My vet worked with me (without my even asking), let me pay for the shots and testing, but didn't charge me for the office visit or the anesthesia and neutering operation. Hope yours will be as generous.
Good luck and keep us posted.
Logan