Poor kiddo, sounds like allergies - I think you're right. If it's a contact allergy, then you're super lucky and can try to avoid the offending plant. However, it's more likely that it's a pollen allergy - and in that case, you cannot avoid it - even if you locked your dog inside the house at all times. Pollens *will* and do get in the house, no matter what.
Some dogs can live w/ allergies, but when they start interfering w/ quality of life and result in the kind of itching that may break skin - that's when you gotta treat it. The choices are:
1) First try an over the counter anti-histamine - you must try it for at least 10 days before judging efficacy - nice ones to try are Claritin or Zyrtec bc these are 24hr pills and are given once a day; if you want to try Benadryl, you should give it every 4-6 hours.
2) After the OTC meds, then you think about this question: Are these symptoms all year round (or most of year) or just seasonal? IF just seasonal, then you consider Temaril-P and/or steroid shots for the season. If year round or most of year, then you consider Atopica or allergy testing/shots.
One thing I'd advise right away - get your kiddo to the vet and have the vet do a skin scrape to see if he has a skin infection already (pyoderma), bc it sounds like he may. The red patches, bumps, and blistery things could be an infection and you'd want to address it ASAP.
Allergies are tough, so good luck. Here is a link about allergies too:
http://www.yorkietalk.com/forums/die...tart-here.html
OH! Before I forget - go to pollen.com and sign up for daily emails for the pollen types and pollen levels in your zip code - this may also give you insight into what else he is allergic too - which will make subsequent years/seasons easier to predict and treat.