My name is***** and I am sorry to hear that Dexter is having such severe skin problems.
When I see an itchy dog, I usually think of the following things:
1) Environmental allergies. Treatments can include prednisone or antihistamines, but can also involve the following- frequent (3x weekly bathing to physically wash off the allergens), fish oil supplements, antioxidant supplements and topical cortisone sprays on the affected areas. You can use prednisone long term if you use a low enough dose, I would aim for about 0.5mg/pound of dog maximum per day. You can also consider allergy testing and allergy shots. These treatments are available through your regular veterinarian or you can consider taking him to a dermatologist.
2) Food allergies. These are also common and something to strongly consider with Dexter. I would recommend a food trial with a hydrolyzed protein diet (Science Diet, Royal Canin and Purina all have varieties of these formulas.) These diets also have a variety of other ingredients that can also help the skin and reduce the severity of environmental allergies.
3) Fleas. Fleas are a very common cause of itchiness in dogs. I would ensure that your dog has good flea control as the first step. Good over-the-counter products include Frontline Plus, Advantage, and the Seresto collar.
4) Mange. This is a much less common cause of itchiness in dogs. Your veterinarian can diagnose this with a physical exam and a skin scrape.
It sounds like your vet already found demodex (red mange) on Dexter. Did they say anything about it being "localized" or "generalized"? There is a genetic condition in some dogs where they get generalized demodex infections because their immune system cannot control the mites. These dogs usually require life long treatments for the mange, such as Ivermectin or Milbimycin.
Here is a link about demodex-
Does this help? What other questions do you have?