Send him to my house. He can eat moles/voles and other natural things. I would not be worried much.
Cats aren't on their menue and the snake will try to escape but if the cat attacks the snake, both could be sorry. Most snakes are not poisonous and are beneficial. The bite from a rat or corn snake or something like that probably would not harm the cat. The two poisonous candidates are water moccasin (was it near water?) and copperhead (I've discounted rattlesnake since you might have seen the rattle). Cats and other larger mammals have learned not to attack the wrong kinds of snakes and each go their own way.

If you see it often try to take a picture and have it identified. Then you can be sure.