The sooner you can take action, the better.
Please follow these tips in this link (www.catsinthebag.org) as they have been successful for others.
Cats are most likely very close by but under a neighbours porch or in a shed. Most cats are found very within a 3-5 block radius.
Put out the litter box and anything that has familiar smells (e.g. blanket) as well as tuna and sardines. S/He is most likely to be found at dusk or dawn as that is when they search for food. If you haven't posted on www.helpinglostpets.com already please do so ASAP as that post goes out to shelters and clinics as well as residents in the area.
You can also create eye-catching posters from the site. Please put them up everywhere and give one to each of your neighbours. You can even give them to school kids and offer them a reward as that will motivate them to search for you. Also, give one to your mailman as they are usually aware of outdoor animals.
If you have a buddy cat still at home, do searches with this cat secured in a carrier. The smell and sounds may attract your lost cat back home. You can also do searches shaking a bag of treats.
Some people have had success with a lit candle on their porch/deck.
Many cats don't come out from hiding spots even if they recognize you so don't be discouraged. They are probably frightened and may not understand why you can't see them.
You can also hire a professional tracking service. Please be sure that they are reputable. Many rescues can recommend one if you are unsure. They may cost some money but they can be invaluable.
Please never give up as many cats are found weeks after going missing. Some even months. And make sure that every one of your neighbours gets a poster and you put posters everywhere. Also contact local vet clinics as someone may bring him in to get him checked for a microchip.