The best thing you can do is watch his yard. The second you see him spread food or put a bowl down - CALL THE COPS. My aunt cared for a bunch of strays, almost 25 catsat one point. A guy who lived down the street from her managed to poison all but one cat within 4 days. She didn't know what was happening, and didn't have the money to take the cats to the vets. (She was living in a trailer park by a highway at the time, and the cats she had were all ones that had been dumped, or who had been left when their owners moved away.) She lost all of the cats because of it.
She found out a month later the guy was known to have left poisoned bird food out.
It's unlikely he'd start with your cats first. He'd want to test out products first, so that if/when he tried it with your cats, it would work the fastest and be something that you wouldn't notice until it's too late.
And remember, poisoned food is dangerous to any children in your neigborhood. Play that card with the cops instead of mentioning your cats. They'll respond quicker.
Edit: If you ever notice him poison an animal, call the newspapers and local news as well. They love any kind of animal abuse stories. It sounds sick, but it should frighten him enough that he won't try it again.