Remedies for Dog's Summertime Itchy Skin?

My 3 year old Australian Cattle dog mix starts itching around August every year then stops with cold weather. It seems to be an overall itch. His skin isn't red or anything, but you can tell he is miserable with scratching. My other dog doesn't have this problem so we're not sure if something in the yard is the culprit. We've tried oatmeal bath and Benadryl - no success. Today it'll be Epsom salts. Anyone have a hint?

By Trudy R.