I have a an 8yr old male cat who poops outside his box at random times. Sometimes next to his food bowl or a foot away from the box. I've had him to the vet a ton of times and it's now been determined that it's behavioral. When I recently moved he didn't not adjust well at all and was even peeing in the house and he's never done that. Now that he has adjusted to his new home he is doing much better. I've also been keeping a "journal" in hopes that I can see a pattern of why sometimes he's good about pooping in the box and sometimes he's not.
Have you changed a work schedule or something like that? Sometimes even the smallest thing can throw them out of whack. I would definitely take your kitty to the vet to make sure there isn't something seriously wrong.
For now your best bet might be keeping him in a smaller area like a bathroom with food, water, litter box to kind of retrain him. Don't get rid of him though...this is his way of telling you that something is wrong!! Also, if you are stressed your cat will feel it too and it can stress him out. I know it's easier said than done but try not to stress too much and maybe it will put him at ease.
Also, there are flower essences you can give them for litter box aversion...that might help too. http://www.catfaeries.com
Hope this helps! Don't give up on him though and hang in there!