First of all, welcome to the board!!
Secondly, I know how upset you must feel, I went through the exact same thing with my little guy. It is very obvious how much you love your little one and want to do what's best.
Have you had a fecal run on him to make sure there are no issues going on intestinally with him? If there is, no food is going to solve that.
If that's all squared away, try giving him a little canned pumpkin. Make sure it's pure pumpkin though, not the pie filling version. That should help firm him up. You can also use Pepto Bismal and Immodium, but it is very, very important to give the proper doseage, so make sure to run it over with your vet first.
You might also try a little yogurt, about a tablespoon every day. The live cultures in that will help with his digestive tract.
And finally, the food itself. I too wanted to feed Canidae. It is absolutely a wonderful food with excellent ingredients, but it is very rich. My little guy's tummy just couldn't handle it. I switched to Chicken Soup for the Dog Lovers Soul. He has done wonderfully on it. I do agree with everyone when I say unfortunately it might be trial and error because different dogs just do differently on foods.
I am SO sorry you're going through such a frustrating time. Hang in there, you will find what works.