Ginger gets benedryl when her allergies are acting up. But I never use it as a preventative. When I start to see symptoms, I give benedryl. It does help her a bit, cuts down on the inflamed skin and itchies. No medicine is 100% free of side effects. Ginger has never seems bothered by the benedryl and doesn't get sleepy. Although, I know others who have given in for plane trips to help their furbutt sleep...so what do I know?
If it were my dog, I'd wait and see. If he starts to show other signs of allergies then give it a try. If you search here there are several threads that discuss dosage...as a matter of fact, there is one I'm thinking of, I'll try to find it.
Good luck!