He defintely needs a skin scrape and a fungal culture done. In the meantime, you can give him Benadryl 2-3 times a day, and that will help him with the itchiness and discomfort. The dosage for Diphenhydramine (aka Benadryl) is 1-2 mg per lb of body weight. So if he's 50 lbs, you can start at a 50 mg dosage. With some dogs, the relief is immediate, but with others it can take upwards of a week to start really having an affect.
If this were my dog, I'd give him 75 mg of Benadryl in the am, 50 mg around lunchtime and another 75 mg dose in the pm. (just space it out evenly)