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) his 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.
Alert shelters,rescue groups,groups that feed feral colonies,vets.
Check regularly with shelters. Don' t rely on their contacting you.
Prayers he is soon and KEPT SAFELY INSIDE.