Put up flyers everywhere asap. It will help a lot. Be sure to tell people to take a picture of the flyer so they’ll have your phone number too. Also people have put their dirty clothes outside for the scent of home.
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.
Prayers he is soon home and safely INSIDE.