Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your pup has sore, purulent areas on his scrotum. These are usually hot spots or superficial pyoderma lesions.
Hot spots are caused by the skin getting wet, sometimes from swimming or bathing and not getting dried off properly but most of the time they come from a dog licking, chewing or scratching an itchy area and breaking the skin barrier due to an allergic reaction of some type, allowing bacteria that normally live on the skin to get in the skin and thrive.
If these are hot spots then getting him to stop licking and allowing the skin to get some air should help in resolving them. This likely involves placing an elizabethan collar so he cannot reach the area and treating the underlying allergy.
Allergies are the most common cause of itchy skin. I'll give you an allergy rundown of likely causes for skin troubles. He may have more than one allergy given how symptomatic he is now. Dogs that have one allergy often develop several with time. The effect of multiple allergies aren't additive, they actually compound one another.
Even if you don't see fleas I do recommend using protection. They are the most common allergen and it only takes one bite a month to make an allergic dog scratch and lick so I recommend using flea prevention even if you never see one again. Frontline Plus, Advantage II or Advantix are excellent topicals, and another option is the new Seresto flea and tick collar that works for 6-8 months continuously. I don't normally recommend collars but this one works very well without toxic side effects for most dogs. Don't use over the counter products, especially Hartz or Sargents, as most are ineffective if not toxic.
Other allergens can be inhaled (like grass pollen, dust mites or molds) and you can use a combination of antihistamines and high doses of omega-3 fatty acids to help with those (they also help with the symptoms of flea allergy). In combination fatty acids and antihistamines work synergistically, much better than either one alone. If his symptoms worsen seasonally I would think that inhaled allergens are a part of his problem.
Antihistamines you can try include:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at a dose of 1mg to 2mg per pound or one 25mg capsule per 15-25 pounds of body weight orally every 8 hours. Dogs take a much higher dose per pound of body weight compared to people so don't let the size of the dose he would need concern you.
OR 2)Claritin (loratidine) at 5mg per 25 pound dog once or twice daily.
OR 3)Hydroxyzine at 1mg per pound orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another. Give the one you pick a week trial and if it isn't working try another. Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight, which means if he is between doses go with the higher one. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day.
Clip the hair in the area with the sores and around his scrotum very short. Use a solution mix of Betadine solution (povidine iodine solution) and warm water, made to look like weak tea to clean the affected areas twice daily. Make sure you get the solution, not the scrub. This is an antibacterial and will dry the lesions out to allow them to heal. No need to rinse this, let it dry on his skin. It does stain horribly though so you probably want to do this outside. Clipping the hair stops bacteria from wicking into the site and allows the skin to breathe and dry.
In cases where the infection gets deeper than the surface of the skin oral antibiotics may be needed to treat deeper infection. If he isn't significantly better in 3-4 days he should see his family veterinarian.
Please let me know if you have any further questions.