First, I urge you make a follow up appointment with the doctor. Finding the cause of the rash is a matter of trial and error. The information that what he suggested didn't help, gives the doctor more info with which to work.
I also suggest that it sounds like eczema. However, only the doctor can tell you that for sure. In the mean time keep his skin moisturized. A daily bath after which you slather his skin with a thick emollient cream. Keep his skin moist by using the cream again later in the day. Your doctor may have recommended some creams. Eucerin is a good one. Walgreens has a house brand that's just as good. We used vaseline on my granddaughter when she was that age. Rub it in good while he's still damp from the bath.
Your son may be allergic to some food, pollen, animal dander, dust, etc. The next step might be to have him tested for allergies. My granddaughter has been taking a prescription anti-histamine since she was a baby for allergies. Allergies and eczema go together. She also has asthma which can be a part of the allergies and eczema but doesn't have to be.
You can take your son to a pediatric allergist. They specialize in eczema as well as other skin conditions.
Later: You said he was treated for eczema. Why don't you think it's eczema? Eczema cannot be cured; only controlled and with difficulty for many. The fact that he still has the rash, suggests to me that you haven't found the right combination in treating it. I urge you to take him to a pediatric allergist or dermatologist.
And he can be allergic to detergents, fabrics, etc. which cannot be diagnosed thru testing.