I don't know. I'd hate to keep a cat inside. It's not something people do in the UK generally. Cats are supposed to be outdoors. I'd let him out to take his chances. Have you spoken to the neighbour? Could you do anything to confirm he wouldn't do it again? Frighten him with legal action? Most people are OK. I feel your pain. It's a real dilemma. We've had cats and they've never stayed indoors.