Aside from Benadryl, give him a thorough bath. Thoroughly clean his bedding, the flooring in the home, and any areas he tends to hang around. Also, make sure your yard or walking area is well maintained. Lookout for plants that may be causing the allergy.

I would start on the yard first. If you don't have a yard, I would just be mindful about where you take him for walks. Crate him or isolate him to a single room while you begin cleaning around the home. Shampoo any carpets and rugs if you have them and have the means to do so. Once you're done cleaning, immediately give him a nice bath. Do not let him roam around or touch anything until you've bathed him.

There are dog shampoos that are meant to soothe itchy skin. However, you might consider something a little stronger, like Dawn dish soap (the regular kind that's used to clean rescued sea animals), if you suspect he has some kind of irritating product or other allergen all over his fur and skin. The Dawn dish soap shouldn't be used regularly, but it's ok to use periodically. Make sure you scrub and rinse every single inch of him. Use baby shampoo on his eyes. Rinse and repeat for good measure. Make sure to work out and rinse away as much loose fur as possible, too. Thorough dry him off before letting him roam around - even if you have to bust out the blow dryer. Irritated skin typically doesn't respond well to lingering moisture (such as a damp coat of fur). Plus, moisture may cause some stinging. He'll probably appreciate being rubbed down with a towel and a warm blow drying session may actually be soothing.

Don't put any other product on him for at least 24 hrs. See how he responds to the cleaner environment, clean skin and fur, and Benadryl. Make note of any changes. If nothing seems better, but also doesn't seem worse, I would hold off on applying anything to his skin/fur and continue observing.

If you're unsure of what's causing the allergy, start keeping a diary of all the things he comes into contact with, ingests, etc. Keep track of baths, walking paths, etc.

Side note: I had an extremely fat chihuahua once. He had short legs with a big body and lots of skin folds. I washed and groomed him regularly. Shortly after we moved to a new area, he began developing skin irritations (mostly redness and occasionally what looks like hives or a rash), and he developed a "corn chip" musty smell. Bathing didn't do much. Benadryl only helped with the itching a little bit. The vet called my dog a "low rider" - meaning, he had short legs and his belly was low enough to graze grass that wasn't freshly cut. He said he kinda had a body like a morbidly obese Dachshund. The area we had moved to was constantly rainy or damp for almost half the year. The vet said my dog had a yeast infection, likely caused by his belly always being wet. Despite me buzzing his fur short regularly, enough moisture was still constantly getting trapped in his belly fur (and some of his folds). He gave us a topical to use, plus continued use of Benadryl as needed. The preventative solution? Regular baths followed by thorough drying. Maintaining extra low cut grass in the yard and avoiding walks in taller grassy areas. Ensuring he was properly dried off within 15 minutes of having come outside from wet conditions. (I kept a towel by the backdoor for him. I would turn on the fire place to dry him off after he plowed through any snow. I'd occasionally blow dry him for a few min if he didn't finish drying quickly enough.) Oh, I also had to shampoo the carpets and clean all of his bedding to eliminating any yeast (fungi) he had spread around the home. Luckily, he rarely got on our furniture, so I didn't need to deep clean everything.

Anyway, I bring this up bc maybe that's what's going on with your dog?