Hey Trigger's Mom!
You might consider a holistic vet - this one is in Ashland: 541-482-6636; this one is in Jacksonville: 541- 702-2288; there are other holistic vets in OR but they are in another zip code. While it is possible the holistic vet may also recommend steroids, they likely will have a greater understanding of how nettles will apply.
Because your boy is allergic to pines and the like, herbal Thuja occidentalis may be better indicated over nettles.
It does sound like your boy has a systemic yeast infection; this would be something to as a prospective vet, their experience with healing vaccine damaged /yeast overgrowth animals.
Consider Ted's Borax Protocol for pets for the systemic yeast; also consider Ted's Mange Remedy or Ted's Anti-fungal/Anti-staph dipping solution to provide some relief for the itchy skin.
Yucca, Tumeric and Quercetin may help with the inflamation; colostrum may help build the immune system - find these at your local health food store. The dose recommended on the bottle is likely sufficient. For the dried nettle leaf you might just mix in 1-2 tablespoons per meal into wet food if you wish to continue that route. Alkalizing, by either adding raw apple cider vinegar to the water or baking soda may also provide relief.
If this were my dog I would use Ted's Borax protocol along with the mange remedy to start, and would also do the colostrum and quercetin. The oral supplements take time to work, while the borax in the water and the mange dip tend to act rather quickly.
Please report back!