If his owner can put his litterbox and/or a worn but unwashed t shirt or pair of smelly shoes of theirs out on the deck this may help lead him home
Go around the neighbourhood at dusk/dawn calling him, rattling a treat container or tap a spoon on his dish or tin of food. Bring a flashlight to pick up the shine off his eyes from under cars/decks/bushes or up trees.
If they can poster the neighbourhood, plus leave one w/ local businesses, schools, pet supply stores, grocery stores, bus stops, vets & all animal shelters in the area that will help garner attention that this kitty has a home who wants him back
Please visit the animal shelters in person often times the shelters are understaffed/over worked with so many animals/requests etc they may not have the time to personally look into each phone call or an emergency/something comes up in the meantime & the request gets overlooked.
Going in person means they can visit & identify the cat right away, if an animal comes in terrified it may act out of character & be labeled as sick or a feral and not be placed with the general population please ask to see these areas as well.
Good luck &