So sorry, OP. I know how attached we can get to them. A few years back I was standing outside with my dog and he took after a another dog who was chasing a deer across our property. Normally he would have been in house or in his fenced yard in back. I did all the things you mentioned: call shelter, lost classified ad, flyers, checking with neighbors but couldn't find him.

So I took out a display ad with his picture (1/4 page ad) on the 3rd page of our local newspaper with reward. I started getting calls of sightings of him and finally found him after 3 weeks. I know it is expensive but it worked for me (the display ad). As far as reward the man who finally contained him until I could get there refused to take the reward.

Also, I used to be an animal control officer. Don't just call shelter. Go down there and look for yourself. We had the dispatcher from hell in the office who also did all the paperwork so the descriptions of animals we had were way off. She'd put down "shepherd mix" for a lab mix, etc. I used to go through her list of animals that came in and would match it to the animal to correct list but didn't have time to do it everyday if I was out in field all day. If I was in between calls and in office and happened to answer the phone for some one looking for their animal I wouldn't rely on list. I went in the kennels & looked myself. But most won't do that and only went by a list. Go look yourself - even if it's for months. By law (in our state) we only kept strays for 72 hours and then our kennel worker put them down due to overcrowding. Sometimes in winter we were able to keep them a few days longer if room permitted. So go at least once every three days (or whatever the law states in your state for how long animal control has to keep it) for as long as you can.

Also, outdoor cats are known to sometimes have more than on household in neighborhood feeding them. There's a possibility that someone within a half a mile of you has made him an indoor cat. Saw that happen more than once.

I'm assuming you kept your cat indoors at night to sleep even though you let him out during day. Many reports of missing cats we had were owners that left them out at night.

One more possibility is that he was hit by a car or otherwise injured and taken to a vet. It wouldn't hurt to call all the vets in your area. And also check with the animal save groups to see if they have him. A vet or some well-meaning neighbour who thought he was a stray may have given him to one of the non-profit animal groups. And don't tell them you lost a cat. Tell them you are looking for a b&w puss 'n boots and go look at what they have. Those groups have been known to not return pets because they figure if you lost it then you don't deserve to have it. Animal control often had to intervene to get a pet back from some of the more radical save groups by threatening them with theft charges.

If you do get him back and for future reference only get the cat collars that have the elastic that will break if the collar gets hung up on something. Also, take the bell off. The bell alerts night predators of where cat is.

Sorry I'm rambling here but wanted to put all possibilities out there hoping it might help.

Anyway, good luck.