Yes. Get him neutered yourself. They will probably never notice. If they do, then deny all knowledge. Problem sorted. The problem is though that he would need to be kept in for a minimum of four days afterwards. Do you have a spare room?
This is one where I fear I disagree with TCN. People have a right to have unneutered cats - daft though it may be - and that's leaving aside the issue of whether the vet would actually carry out the procedure and/or any possible consequences for them or you.
I would approach the neighbour first off and explain the situation.
I think that's like saying that people have the right to abuse their children. I think the animals' right to treatment for the good of their health, which neutering is, trumps the owners' rights to be dickheads.
As for the vet. You obviously don't say he has owners. You say he is a stray that you are prepared to take care of once neutered. And it is always a reasonable assumption that any unneutered male is in fact a stray. So no one could argue with that assumption.
Don't say you know who he belongs to if you want to get him neutered. Say he's a stray who has been hanging around, and is is possible to neuter him.
If he has a microchip (unlikely I suspect) they will contact the owner first.
Here, the local cat rescue will come out and collect un-neutered cats, neuter them and rehome them. They work on the principle that if a cat isn't microchipped or wearing a collar, and isn't neutered, they are feral/stray.
That's just my view. I don't like the notion of carrying out some sort of pre-emptive strike on another person's animal. I appreciate, TCN, that you see daily the tragic consequences of not neutering but that approach has always made me uneasy.
I don't know if they can do anything for you - we had a problem with toms (about four of them, over the years) and Kildare Animal Rescue have come and caught them for us (for a donation), got them neutered, and "rehomed" them in farms.
I think they were feral, I don't actually know. What I do know is that they made our lives miserable, coming in the kids bedroom windows and spraying everywhere [bleurgh]. I couldn't put any washing on the line for six months at one stage - in fact it was showing the Animal Rescue woman the washing that made her stop asking questions and just catch that one
I'll get in contact with them and see what can be done.
I got help from Cats Aid a few years ago because there was a stray (that I fed) who was limping. Got a trap off them and took him to the vets. Turns out it was a tumour and he had no teeth so he got put down.
Funnily enough my next door neighbour thought at the time that that cat belonged to this man. I didn't think so at the time (this Tom was about at the same time and they looked similar except ones black and white, and the other was grey and white).
Though the stray was neutered, and I wasn't aware the Tom wasn't, I'd never seen him spray and i hadn't a cat for him to attack