Cats are amazing survivors so don't get discouraged. The GOOD NEWS is that unlike dogs, cats normally do not roam very far from home ... WITHIN a 5-7 house radius. Your cat is probably scared, but hiding nearby. Just know that after being outdoors for awhile, an INDOOR cat will go into a protective SURVIVAL mode and will probably NOT respond to YOUR calls ... even if nearby.
If you have a GARAGE, leave the garage door slightly OPEN so your fur baby can have safe access to return home. Place items with your kitty's scent along with STINKY wet food (tuna/sardines) and WATER inside the garage to help lure your kitty. If NO garage, try to leave those above items outside NEAR the door where your kitty EXITED your home.
Cats feel safest to come out of hiding when it's QUIET and DARK. One suggestion would be to walk around your neighborhood at night (between 10 pm - 2 am) shining a FLASHLIGHT and look for reflecting EYES ... beneath cars, bushes, decks and on people's doorsteps. As you walk with the flashlight, call softly for your kitty and SHAKE your car keys or cat treat bag ... a sound that your cat will recognize. It took thirty (30) days to find and rescue my friend's cat. She was hiding in the STORMDRAINS during the day and coming out at night. It just took forever to get any sightings of her.
Please ask your neighbors to CHECK inside/under their BACKYARD SHEDS, decks as well as their GARAGES. Cats have been known to get LOCKED inside a shed/garage after owners finish yardwork.
Also visit your nearby animal shelters IN PERSON. Don't just call as your kitty may be overlooked by the busy staff, so actually go there. I recommend visiting EVERY OTHER DAY, because after the required holding period, the shelter can legally adopt her out or worse (if injured/sick).
I hope and pray that your sweet kitty finds its way back to you. Please keep us posted.
Put his litter box on your porch, along with an article of clothing with your scent on it. Cats have a wonderful sense of smell and will come to those items. Leave a window cracked nearby so that you can hear him. Post pictures around the neighborhood and on doors. Ask neighbors to keep an eye out and to check garages and sheds. Cats hide, usually fairly close by. They tend to come out after dark, so look for him then. Call and rattle a treat can if he likes those. If you put food out, stay close by, because it will also attract other animals that might scare him away. Don't leave it out very long. Also, go in person to shelters and vets' offices, and leave a picture and phone number there. I hope you find him!