Please try to find him a home if you're capable, I understand not being able to do anything, but, cats are destroying the ecosystem and you're, as heartless as I know this sounds, helping an invasive species wreck local species.

I would never advocate bringing harm to a fellow creature, I don't even kill bugs that aren't parasites, and I wouldn't suggest you take them to a shelter because I know how cruel the conditions are, but please, whenever possible, try to make them an indoor cat, and do spay and neuter any cats whenever possible.