Go out at night with a flashlight & a bag of treats. Look under bushes, porches, decks & up in trees. You may see the reflection of his eyes. Do NOT put out the litterbox. It may draw other animals who will mark & scare your kitty away. Put out a blanket or towel that smells familiar. Good luck to you.
Put a poster on your lawn and lots of poles. Give one to your letter carrier, local PD'S, vets, shelters. Check with shelters daily, in person if possible. Go out after dark with a flashlight and look under sheds, decks etc. Ask neighbors to do the same.
Best of luck