Skin issues can be extremely irritating to patient and owner. Sorry your little guy is affected by this.
There are two or three things I think about in cases like this. The first is an infection with yeast organisms. These lesions are often black, have really thickened and greasy skin, may have a little sweetish, yeasty odor to them, and are very itchy. Yeast infections are generally considered to be secondary issues to some other problem - like allergies or Cushings disease or some other problem. So without controlling the secondary issues, the yeast infections are often recurrent or difficult to eradicate. OTC things you may try for this include gyne-lotrimin antifungal cremes and Selsen Blue shampoos. Make sure to leave the lather on for 10-15 minutes before rinsing for maximum effect. Unfortunately, these often require oral antifungal drugs to get rid of the bugs, and prescription shampoos work better.
The second thing I would consider would be chronic, recurrent bacterial skin infections. These are also often secondary issues. But if you don't control them, even if you address the primary issue, the dog remains uncomfortable. So if this is the case, he needs antibiotics - sometimes several weeks worth.
The third thing would be sarcoptic mange. This is certainly less common, but the mites that cause this are difficult to find on scrapes, so negative scrapes do not rule out sarcoptic mange. In cases that I suspect it, I will treat for it even if I don't find the mites.
In this breed, I would certainly recommend ruling out Cushings disease as an underlying cause of chronic infectious skin diseases - especially skin infections. This is done by blood work which your vet can discuss with you.
Hope these things help. If so, click the ACCEPT button.
I forgot to mention that I have also been using a acetic shampoo which I got from my vet. It seems to help very short term but within 24 hours he starts itching again. When I wash him, little pieces of what looks like skin come off until he is left with just a big red area. These spot are only on his undercarrage and on his backside, not on his back or legs. I'm not sure if trying the Selsen Blue or Gyne-Lotrimin will help since I'm already using this acetic shampoo?
What should I ask my vet to test for and in what order? They love to charge the hell out of me for tests and we haven't had anything that has seemed to help.
Also, is there anything OTC that I can give him orally without a perscription to help?
Whether or not the gyne-Lotrimin will help depends upon whether or not it is yeast. If your vet has not done skin cytology to determine whether yeasts are present or not, I would do that. The acetic shampoo is a good general purpose shampoo. It may be as good as the Selsen Blue and may have a broader spectrum of activity. However, if it's yeast, the best shampoo is a ketoconazole shampoo which is prescription in our hands. I'm not sure there is one OTC.
If the cytology is negative for yeasts, but positive for lots of bacteria, I would recommend appropriate antibiotics for an extended period of time. It may require 3-6 weeks. I would also be tempted to treat empirically (without a defiinitive diagnosis) for sarcoptic mange just to cover that base.
The test to do for Cushings is a routine screening blood panel. It will raise red flags that indicate further tests need to be done to rule it out (ACTH response test or low dose dexamethasone suppression tests). The other signs of cushings includes increased water intake, increased urine output, increased appetite, pot-belly, skin issues, poor hair regrowth after grooming or clipping, and others. If the screening bloodwork does not raise red flags and you have none of these signs, then cushings is unlikely.
The only other OTC thing that I would recommend is the supplementation of omega 3 fatty acids which are helpful for skin health. You can get these by giving fish oil caplets at 2-3 per day for your dog.
Good luck with this frustrating situation.