Put his litter box outside, under cover, along with some really smelly cat food and some clothes you wore recently to attract him. Check carefully around the outside of your house and your neighbors’ houses. Cats don't usually go far initially and they can tuck themselves into unbelievably small and unusual spaces. One person found her cat hiding in the grill on her patio after searching for three days. Go out late at night when it's quiet and call him. Make large, easily read flyers and post them around your neighborhood with the cat’s picture and your contact info. Post a lost ad on nextdoor.com. Ask your neighbors to check garages, sheds, under decks - anywhere a cat can hide and get shut in. Borrow a trap from a shelter or vet. Don’t give up. Good luck!