If this is an allergy, one of the best things you can do is to clean the dog after he goes outside. You need to get the pollen off of him, so you can clean him after each trip outside. That can be a huge task, rather overwhelming, so at the very least he should be cleaned up each night before bedtime so he isn't sleeping with the pollen all over him. They sell pet safe baby wipes at pet stores, but the prices are nuts. I would find a hypoallergenic sensitive skin fragrance free baby wipe (Walmart has a good one in their Parent's Choice brand) and wipe him down with a couple of those as often as possible. Our vet uses these on her dog and we have been using them on Captain Morgan for quite some time. Any bedding the dog sleeps on regularly should be washed often to keep pollen off of it.
Benadryl can be super helpful, but be aware that it can make them drool excessively. A cat we used to have drooled so much he looked like he had rabies or a Santa beard each time we gave him liquid benadryl. The pill didn't cause this, but the liquid benadryl was astonishingly good at causing this. Not all cats or dogs do this, of course. It scared me to death the first time it happened.
Oatmeal baths can be very very soothing, but PLEASE don't waste your money on the Aveeno packets. They are incredibly overpriced. You can put either old fashioned or quick cooking (NOT instant, NOT flavored) oats into the blender or food processor and process into flour, then dump that into the water and bathe the dog. If you don't want to process the oats and send them down the drain, you can put the oats into an old sock or section of pantyhose, tie a knot in the end and toss that into the bath water. Let it soak a couple of minutes, squish it around with your hands if you want, and when you are done just throw it away. For a bath in the tub, I would use about 1/3-1/2 c oats. If you use a smaller tub or a sink, use less. If fleas are a problem, a bit of lavender or rosemary in the tub is helpful at getting rid of them. You can put dried lavender or rosemary into the hose with the oats if you want, or you can just toss a bit into the water.
Baking soda in the bath water is also very soothing, though I haven't ever used it with oatmeal. I have added a few drops of lavender essential oil to a container with baking soda to make a bath treatment that is great for humans and pets.
If he gets one spot and licks and chews it until it is raw or bleeding, see the vet. Often the vet will give a topical treatment and the problem with that is the dog licking the medications off or chewing off a bandage and then licking off the medication. You can get something called Bitter Apple at places like Tractor Supply and Atwoods. It is a nasty tasting liquid you spray on an area to keep the animal from licking the area. It can be very effective at stopping the licking and chewing. We always made sure there were no cuts or sores in the problem area before we put the Bitter Apple on. If there are sores/wounds, apply the stuff to the outside of the bandage, NOT the skin.
You might also consider using a bit of oil on the dog's skin after each bath. I would use rice bran oil probably, or maybe almond oil if almonds are safe for dogs. Rice bran oil would be my preference because it is really light and absorbs super fast. It is also super gentle on skin, and safe for the dog to lick. I greatly prefer to use food quality oils rather than commercial moisturizing products because they are far safer than all the chemicals in commercial moisturizers. I get rice bran oil at Big Lots and it isn't expensive there - $6 for a 32 oz bottle.