I don't understand why they don't neuter him, there are places that can do it for free, I feel for you as he will also be spraying everywhere too, as you prob know it's an awful smell, it also calms them down and is safer for the poor cat, as it'll be fighting too, I can see why you want to feed him, but I'd not be letting him the house though, it's not fair on your elderly cat. Maybe as others have said have a chat with them, hopefully there approachable People, I wonder if theyve had cats before? Anyway good luck, just for info I have 2 beautiful neutered ginger boys 🐱🐱