With the number of autoimmune issues that GSDs have, I have always been careful not to recommend immune boosters like echinacea. If there aren't immune system problems like pannus or IBD, you could probably use it. I don't know about the dosing though.
There is sticky in this section that has other allergy suggestions.
I agree with the other responses. Echinacea is not an appropriate choice for allergies...perhaps freeze dried stinging nettle but while there are other herbs I might suggest for allergies, an herbalist or similarly trained practitioner would need more information to evaluate the cause to suggest remedies for your dog's individual physiological make up. Diet is the main area I would look to discover sources of allergens followed by prior vaccine history and current vaccination practices.
I also use it at home for myself-firm believer in it. But I don't see how it could help with allergies for our dogs. It is an immune booster for stuff like infections..colds etc. I can't tell you how many colds I have warded off by taking it!