Once dogs get the taste of raw, they can become picky about dry food.
There is no absolute correlation between diet and tear stains. Meaning, A follows B kind of cause and effect.
Apple cider vinegar is said to help balance out the blood pH and help with tear stains.
I have been adding this to my EB's raw diet, but seeing no improvement on tear stain issue.
Regular wiping (once/twice a day) is the only recourse I have now.
Regarding supplements (not related to tear stains):
Is there enough bone/calcium content in Winston's diet?
If mince is meat only, he needs a good source of calcium. Bone is the first choice.
Calcium supplements, in the form of pills and powders may be not be easily absorbed.
X mg of stated calcium may not result in X mg absorbed calcium.
Bit of organ meat is also good; small quantity of chicken heart and liver to start with.