Put out his litter box,smelly food,pieces of your clothing.
Make a chum trail with used litter or wet foof in a gallon of warm water.
Check consistently garages,sheds,basements,secluded spots,under porches and vehicles. One of mine ( indoor) hid 5 days in a neighbor's basement. Had looked there several times.
Go out at night with a flashlight and call him.
Go repeatedly,especially late at night.
Get a humane trap.
Alert shelters,rescue groups,groups that feed feral colonies.vets.
Prayers he is soon home and safEly INSIDE.