Still worried about skin problems

You can (and should) double the dose of the benadryl.  If he's 50 lbs, he should be getting 50 mg, 2-3 times per day.  Upping the dose might help quite a bit.  Did the vet give you any type of medicated shampoo?  If not, you might try finding something hypoallergenic with Aloe in it and see if that helps with the red rash.
 
Did the vet take a culture to see what was on the skin?  If not, you might give them a call back and let them know that he's not better.  If they don't do a culture & sensitivity, they're really just guessing at what antibiotic to prescribe.  I know they often use Keflex because it's the most broad spectrum antibiotic for skin infections, but the last 2 infections my lab had would have never responded to Keflex.  Fortunately they'd done the culture & sensitivity, so we knew exactly what to use.

    He should get 50mg of Benedryl twice a day; here's a link that has different types of antihistamines for dogs with dosage by weight;      [linkhttp://www.utskinvet.org/pdf/antihists.pdf]http://www.utskinvet.org/pdf/antihists.pdf[/link]      Benedryl didn't seem to help Jessie, but 1 Tavist twice a day did some good; just make sure it's not the TavistD. Walmart's Dayhist is the same as Tavist but cheaper. Have you tried bathing him? Jessie has been itching more lately too and the vet has me bathing her once a  week with a good skin soothing shampoo and a leave in conditioner with hydrocortisone. You may need to try a short low dose round of prednisone to calm the itching down while you  try more Benedryl or Tavist, and the baths. He may need a longer course of antibiotics; skin infections are hard to clear up and many vets don't treat them long enough. Because of the blood supply to the skin, only about 40% of the antobiotics given reach the outer layers of the skin, which means a dog may need to take them longer for a skin infection then for other types of infection. If you live in the southern part of the country and the rash is on his stomach and legs he could be suffering from a contact allergy. A common offender is Wandering Jew, which is a vinelike plant; St.Augustine sod is another one that causes itching. Have you taken him back to the vet; maybe he needs a different antibiotic to clear up the infection.