Hello, I am Dr RB and I am a small animal veterinarian. I live in the southeastern US so I see ALOT of patients with skin allergies! The path for an itchy skin includes ruling out flea allergy first, then food allergy, then inhalant allergy (e.g atopy). Some dogs have more than one of these but hopefully not!
As far as flea allergy goes, there is a common mistake make by pet owners in thinking their pet has to be infested with fleas in order to have a flea allergy. However, this isn't the case in dogs that have a severe allergy to the saliva in fleas; one flea bite may be all you need. This includes sand fleas so rule this out first and make sure he Darrell is on a monthly flea repellant/preventative (my favorite is Seresto collar) before going any further.
Once you rule out flea allergy, then rule out food allergy first. The very best way to rule out as well as treat a food allergy is to use a prescription hypo-allergic food prescribed by your veterinarian. Examples include Hill's Z/D food or Royal Canin Hypo-allergenic diet. A true elimination diet for 8-10 weeks (not including the first week that you are mixing the food to prevent GI upset) would be required though. This means no treats or anything else or you would need to start over! Your veterinarian could help you this if prescription if needed.
Inhalant allergy or atopy is usually to environmental allergy such as mold, grass, mold, pollen, etc etc. Diagnosis can be determined with a blood test or skin test and "allergy shots" can be made specifically for your pet to help them during allergy seasons.
Regardless of the cause, most severe allergy that causes excessive itchiness usually leads to a secondary bacterial infection. This may the case in Darrell is his skin is already showing signs of bumps, especially if skin is red or exudative. In this case, he may need an antibiotic to get rid of the skin infection before he can tackle the allergy.
I hope this advice is helpful to you! However, let me me know if you need any clarification on anything. If my response was helpful, please take the time to rate my answer today. Let me know if I can help further with this:)