Another Peeing Problem?

I don't know what else I can try with my cat. He constantly urinates in the corners of our home. I have tried the following:
1. Changing his litter type (we have 4 cats, 5 litter boxes)
2. Cleaning up the mess with Nature's Miracle
3. Putting items in his "favorite spots" (he always finds new ones)
4. Taking him to the vet for urinanalysis (3 times)
5. Feliway (for possible anxiety)
6. Prozac prescribed by the vet
7. Valium prescribed by the vet
8. Confining him to one room
9. Giving him extra love
10. Installing hardwood floors (maybe he wouldn't pee because he wouldnt' feel like he could bury it?)
He is STILL doing it. The only other option I can think of is trying to make him an indoor/outdoor cat. I really don't want to do this as he is 4 and has always lived indoors. I want him to live a long life but I just want him to STOP. Anyone have any luck with this?
Answers:
We had one cat (of four) that did this relentlessly. Finally I gave her to my brother -- a single-cat person -- and she's been a perfect litter-box kitty since. She was a very social kitty, never fought with the other cats or anything, but apparently being around other animals made her "need" to mark her territory.
Of course, living with her encouraged my beloved 12-year-old male cat to start peeing on the floor. . . .

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Has he been neutered?

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if there cats outside peeing than he is trying to mark if there are other cats around

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some cats are just very picky about thier litter box, how often are they cleaned? where are they located?
several factors can affect inappropriate urination
1. dirty litter box- how often do you clean it
2. location- are they all in the same room,
are there any loud noises in the vacinity
it is on cold floors (cats do not like walking on cold floors)
does he have easy acces to it?
3. is he neuterd ( when he does not use the litter box is he really urinating, or is he spraying ( his scent plus a little urine around?)
the cleaning of the litter box would be the thing I looked at first