Put out his litter box.smelly good.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.
Put up lots of posters.
Alert shelters,rescue groups,groups that feed feral colonies(Animal Allies ). vets.
Prayers he is soon home and safely INSIDE.