Keep checking 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.
Make a chum trail with used litter or wet food in a gallon of warm water.
Alert shelters,rescue gt9,groups that feed feral colonies vets.
Prayers he is soon home and SAFELY INSIDE.