I have an 18 month old cat, I've had him since he was 5 weeks old and although he's never had any reason to be?

frightened of strangers, he's petrified of them. He's not an outdoor cat, and he has a playmate. The other cat is almost entirely opposite. If anyone he doesn't know comes into the house, he hides for absolute hours, and it's worrying me. What can I do to help him?