Welcome to the forum. My last dog was a Weimaraner (and currently have a Vizsla). Never seen a condition like this. Hope the new diet helps.
I do need to argue against following the advise to feed chicken backs as stated in the Getting Started Guide. Feeding backs—without adding a substantial amount of boneless chicken to compensate—will result in your dog getting WAY TOO MUCH BONE!!!
I'm with TRDmom on this. You will likely get a dog that had GI upset, vomits, or has constipation if you feed backs alone. I'll be less diplomatic and say the Guide is not only not "the holy grail," but is very controversial and IMO offers very poor advice for getting started. I wish this Guide would be revised, as the bone is backs is more than 400% of PRM ratios (so if widely out of specs with the "model" this forum supports). There are downsides to feeding this mush bone, and it is unnecessary.
Chicken quarters (or drumsticks and thighs) have plenty of bone for starting bone-heavy. They are a far better choice than backs IMO.
TRDmom makes a good point about making sure the chicken is not "enhanced" (injected with more than 75mg of sodium).
Also I'd advise stripping the skin from the chickens initially (save it in the freezer) and then build up the amount of skin left on as stools prove firm. Dogs need to condition to fat burning (which is optimal for them) so too much fat right out of the box can cause loose stools (which is why some advise over-dosing the dogs on bone to compensate). Better to reduce the source of loose stools (too much initial fat) and feed bone closer to PRM percentages. You DO want to build up the fats. Fat is essential to good health and high energy. Dogs just need to adjust many bodily functions when converting from carb burning to fat burning.
Best wishes with your W.