Poop Shy?

Question
Hi,

  We have an almost 15 month old black labrador puppy (Sam) who thinks it is absolutely ridiculous that we ask him to poop outside!  I have worked and worked and worked with this guy and he is just stubborn about it.  I have gotten him to pee about 95% outside, but he just won't do it outside.  I'm averaging at about 1 poop outside a week!  He knows that we want him to do it outside but he seems to think that this is just not an option.
   He's very shy about it, he doesn't like anybody to see him do it, so I have gotten him a tie out and will go back inside to give him his privacy and I watch him from a window I know he can't see me from to keep an eye on him.  The few times I see him go it's almost comical because he will stoop and then he will look around, stoop, then look around, then finally go!
   You probably want some background on him.  We rescued him from a local shelter when he was almost 3 months old.  He was on his last day, he was going to be euthanized that evening.  We fell in love with him immediately.  He developed seizures the third day we had him and he has severe allergies that gives him itchy skin.  We have almost gotten that under control, we're trying a new dog food (4th kind) and tonight he had to go back on steroids, an antihistimine, antibiotics, and he's always been on his prednisone for his seizures.  He's a brilliant dog!  He sits and shakes better than my other two that are 3 and 6!  He makes wonderful eye contact, I can tell him to go open his door to his crate and he will run in there and paw it open.  We knew potty training was being set back because of his health conditions but we never believed that it would be going on past a year of his age!  He is neutered by the way, that happened at just under a year, we had to keep delaying because of his skin problems.  
   He's now on California Natural dog food, that is fish and sweet potatoes.  The vet told me she now thinks the potty training has gone into a behavioral issue and she hasn't any ideas to help us!
   He will simply hold it until he's able to get back inside.  We've tried keeping the spare bedroom door shut, (that's as we call it HIS bathroom, least he thinks so!) And he whined and cried and howled at the door, and we would take him outside and tell him "Potty here" and he would look at us with the most disgusted look on his face!  I eventually had to pull up the carpet in that room and am not putting anything new down till I have this issue under control!
 I have tried walking him, he'll wait till he gets home.  I've taken his poop outside and placed it in the yard and showed him "this is where you potty" he looks at me disgusted.  We can play outside for over an hour soon as we come in, sneaks off to poop!  I've tried crate training...holds it till he can poop in another room.  When I really stand firm with him it literally stresses him out so much he gets diarrhea and then it's just impossible to get him outside in time!! Putting him on the tie out seems to be the most promising, but again he'll go about once a week which we praise him like crazy and give him treats and he knows he did good!  He's a smart dog!  But he is just determined that he is to go in the house.  If he can't sneak to the spare bedroom he'll sneak into another bedroom and go.  
  We have two other labs a 6 year old neutered male yellow lab/golden retriever mix (Happy) and a 3 year old spayed female black labrador (Lucky).  Neither of them have this problem at all...well the female has some anxiety issues that are steadily improving with training and if she hears a motorcycle she will not want to go outside for the rest of the day, she is just deathly afraid of them and sometimes she will have an accident because she won't go outside and just can't hold it anymore she's actually lost a lot of weight because of this...though now we're getting her to gain some back thankfully.  We've been having good progress with a friend of ours who has a motorcycle come over and just sit on his bike with it off and feed her treats.  Other than that no accidents from either of the two older ones.  When Sam (the almost 15mo old) see's them go outside I make a big deal of telling them how good they are so he can see they go outside and are good about it...but to no avail!!

  Anything you can offer us would be great!  I'm running out of ideas...and floor cleanser!   Let me know if you need anymore information about our three.

Answer
You have some good ideas going here, I'd like to extend some of them. I can make some guesses as to where this behavior came from, but reasons aren't solutions.

First, you must mark the behavior right when it happens, rewarding them after they come back inside doesn't do any good, it just rewards them for coming inside. Let him see you reward the other dog right at the moment the dog is pooping. And if you he's bound and determined not to let you watch, clicker train him, then hide and watch and click when he poops. He'll be able to hear the clicker from some distance, even inside if you make sure the window is open. This will mark the behavior right as it happens, which is the only way to convince him that that's really what you want him to do.

Other than that watch and confine are the magic words. Keep him in his crate at all times that you can't have your eyes right on him and when he's hanging around outside the crate, keep is leash on so you can get him outside quick. If you catch him going in the house, make a noise to distract him and then hurry him up outside. Don't scold or punish, there's no point and don't try to punish him for old poop. He's just not going to make the connection you want him to and he'll just learn to be more sneaky.

Visit http://www.thegooddogjournal.com/2008/11/housebreaking-made-easy.html for the basic method I suggest for all housebreaking and adapt it to your needs with the suggestions above.