Hello and thank you for using this site to help you. Even though you do not see any fleas, this could still be related to fleas. A flea can hop on, bite, and hop off or die from the bite, but still cause an allergic reaction on a dog because the dog is actually allergic to the flea's saliva. It is much like getting a reaction to a be sting, except it causes an uncontrollable itch rather than swelling. I will direct you to a link that goes over this.
At this point you want to stop him from getting to the area because a dog's mouth is full of bacteria and this biting can cause a bacterial infection on the skin.
Try using a cone collar to prevent the dog from getting access to the site. Keep the area clean by washing it and drying it well and then put a bit of a triple antibiotic cream on the area. As it starts to heal you can get a cortisone rub for dogs (found in a pet store) and use that to help with the itchy feeling.
Another possibility for chewing is stress. If this is the cause then you can try some over the counter melatonin to reduce stress, (as long as the dog is not on other meds and is of good health.) Put the cone on so the dog does not have access to the area and interrupt the behavior and redirect the dog to another behavior. I would keep things like Kong toys that can be stuffed with cream cheese or treats or dura chew nylabones, so the dog can concentrate on something other than chewing.
dosing on melatonin (found in vitamin aisle of the drug store)
You do have to be careful that all the chewing has not left such a bacterial infection that the dog now needs oral antibiotics from your vet.