Put out his litter box.smelly food,pieces of your clothing.
Make a chum trail with used litter or wet food 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.
Check with them frequently.
Prayers he is soon home and safely INSIDE.