I have in the past tried the Feliway plug-ins, but can't say I noticed they had an effect on my behaviorally-challenged cat one way or another. For me, it was pretty much a waste of money.

Varies from critter to critter, so you just have to try. Random responded marvelously to them, as does Goku (to the dog version).

I think I might still have a couple of spare diffuser units, which is the expensive part. Drop me an email if you'd like me to look and I'd be willing to pass one or two along depending on how many I have. Our dog Totally Freaks Out (lots of OCD-like behaviors and he completely forgets his housetraining, usually complete with vile diarrhea) when we move so with this last move we were begging, borrowing, and mooching the Comfort Zone units from everywhere and everyone we could so we could put one in every room since our new house has so little circulation between rooms.

The refill bottles are cheap on eBay if you're willing to accept ones that are past date. I contacted Farnam to find out if the product would spoil or be less effective and they said no, they're fine for up to a year past date. (Sorry, since my kitty-girl crossed the Bridge over 2 years ago I don't have any cat refills on hand.)

Other suggestions:

fragrance-free litter
sand-like texture vs. pellets, crystals and such
cover-less box
very large box, like a big sandbox
extremely clean box at all times
multiple boxes, one to call his own, if there are other cats
*huge* amount of potty box praise when using the box

Re large boxes, especially, could he be having trouble jumping in and out, due to old age or arthritis?

Any windows, doors, etc., down there where he has seen outdoor kitties that might be coming around, ie, he's marking his territory?

Not getting along with any other cats that he might have to share the box with?

Could even be that he doesn't like walking on the clumps in clumping litter. I've got one that seems bothered by that when the box isn't clean.

Squirrelheaven - Everything you mentioned is a possibility. I've tried all sorts of litter and I am trying to be more diligent about cleaning it, but the funny thing is, he always poops in the box. He may urinate in there too, it's impossible to know since I have two cats. I bought fragrance free today because the smell of the fragrance combined with urine makes me want to gag. He will not potty if I am anywhere near, so praise is out of the question, although he will urinate on the wall/window with me watching and screaming at him. Go Figure. Maybe I should call a pet psychic.

Johnmari - what a sweet offer. I will shoot you an email. I'm willing to try anything.

Sounds like he's spraying to me. I've got a neutered male who does this all over the garage (he's an outdoor cat). We're not sure why, no other cats go in there except his long-time female companion cat.

Can't speak to the pheromones, never heard of them, but I'd sure be interested in learning more about the dog ones. Our neutered GrandPug pees in our house every time he comes over. It's a dominance peeing and it drives me bonkers.