Sounds like he might have one of several skin conditions causing this. Best thing to do is see your Vet and get the proper medicine to help him. My Basset has that problem here and there and the Vet gives me a prescription and it stops within a few days. Don't let the dog suffer, call your Vet.
Could be any number of skin conditions .. Caused by something like a food or environmental allergy, or even an insect bite. Until you are able to get him to the vet to have it diagnosed so you can treat it .. Try giving him a Benadryl (antihistimine) and see if it provides any relief. I believe the usual dosage for treating dogs is 1 mg/lb every 8 hours (three times daily) but can be doubled to 2mg/lb if needed.
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