You need to determine the cause of the itching before you start giving meds. The dog could have anything from a bacterial infection or fungal infection to a parasite or an allergy. Sounds to me like it might be time for a vet visit. You don't want to give unnecessary medications that are not going to help anyway.
But to answer your question, if a vet determines that your dog has allergies, you can give Benadryl WITHOUT decongestant. 1 mg per pound.