Very picky dog, can't find food he'll eat!

Little Bear is driving me crazy. He has never been picky about his food until the last few weeks. It seems anything we try to give him he either won't touch or he'll eat a few bites and leave it.
We've fed him gravy train before and he ate that great, the last time we bought it he wouldn't eat any of it. So we tried Alpo Bits and Bones or something like that, he takes a few bites.So I tried adding some canned food to his dry, nope, he didn't like that either.
He is kept in a large pen by himself because right now the puppies are too young to leave outside so the momma dogs have to be out with them so he has been alone for a while. When I am outside for any length of time I let him out to run and play but he can't stay loose.
Due to some issues with ex neighbors he now thinks all strangers are going to be mean to him so he gets defensive and has to be kept in the pen or on a run. He has 2 large dog houses to go in when he wants to but I think he gets lonely and I'm sure he gets tired of being cooped up while all the other dogs are free to run around the farm. He's lost some weight and we've switched his feed atleast 4 times since all this started. The other dogs aren't really picky.
I've decreased how much he gets fed by half because it's pointless to offer him a lot that just gets spoiled.

I can relate. My female will eat anything. Our male is very picky. I jokingly say he is as picky as the breeder in Germany I bought him from. We finally found one he likes but it's on the pricey side. He is getting Blue Buffalo our female gets Diamond Hi-Energy hunting dog formula.
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Honestly, neither alpo or gravy train are doing any good for your dog. Try Tractor Supply for Taste of the Wild, or Canidae or Solid Gold. You will feed less, you will have less waste and a happier, healthier dog.
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Normally I would say remove the food after ten minutes and then he doesn't get another chance till the next day.

However, if this began around the time he started being kept in the pen alone, it sounds like a case of depression and not a picky eater.

How often does he come out of the pen? Dogs are social animals, I would suggest he be out at minimum twice a day for two long walks. Once in the morning and once evening. More if you can.

If he's not used to being alone for extended amounts of time this is going to take a toll on him emotionally. The refusal to eat is not the problem, it's only a symptom of the real issue, which is the dog being depressed.

Only way to remedy this is to get him out more, exercise him more and make sure he has ample chance to interact with people and your other dogs.

My dog is not as bad of a picky eater like yours, but he IS picky.
However, I feed him Blue Buffalo, and he eats properly now..

I sometimes add a banana or yoghurt to his kibble too.
I also pretend I eat it, so that I'm in my 'Alpha' role.
For me, that helps! Because what 'Alpha' eats, must be good!

Coffeeaddict, he gets out atleast 2 times per week. He gets out more when we're outside messing with the horses or something. We can't just leave him outside because of the joggers. If they'd just talk to him and pet him instead of throwing sticks and stuff at him he'd probably like them a little more. He's a sweet good dog but he has a protective instinct. If we are out with him he's fine once we tell him it's Ok. He listens very well also.
If I mix the dry food with a little left over cooking grease or add some scraps to it he'll eat it better.
I also think he hates being penned all the time. He used to be able to come in the house quite a bit but we've been staying at my parents in a camper until we move this weekend so he'd had to stay either on a chain, a run, or currently the pen. When we move we're going to fence in the entire backyard for the dogs and he'll be much happier then.

Are there any good dog foods that put weight on a thin dog pretty good? Our great dane had puppies 3 weeks ago but she's always been thin. We've dewormed her before she got pregnant but the only time she gained weight was when she was kept in the pen also. She has a lot of energy and I think she burns off food as quick as she gets it. She gets fed twice a day, about 1 1/2 average coffee cans per day. I don't like seeing ribs on dogs but everybody else is at a good weight.

What are you feeding the dane?

I would seriously look into some of the foods mentioned (blue buffalo, TOTW, etc) for all your dogs.

That's the problem. Twice a week is not enough. I'm not suggesting you leave him out alone, I'm saying you should go out with him, walk him, run him, bike him, whatever.

Twice a week is not enough. Twice a day is what you should be aiming for. Until you resolve the depression from being ignored, the eating problem won't go away.

That's the minimum, usually it's more than that. He isn't exactly ignored even when he's in the pen because I'll usually brush him and play fetch with him for a little while when I take his food out and just about every time me or my husband is outside we let him out to run and play. I can't even find any Blue Buffalo brand dog food here and our budget doesn't allow for expensive dog foods.
We feed our Great Dane either Alpo Bits and Bones, Gravy Train, or a high performance brand that has higher levels of protein and we feed her half a big can of Ol' Roy we mix to her dry food. Our other dog, a little terrier cross eats the Bits and Bones good but she's not that picky either and the Dane will eat anything.

You have a TSC near you? They carry both TOTW and Blue buffalo. In the long run you will probably need to feed less of the good foods versus the very large amounts of crap (and yes they are crap foods) food that you are feeding.