Alergies?

What's the best way to treat dog allergies? Sonny is licking his feet and scratching non-stop for the past two weeks... He doesn't have fleas- I've checked him everyday for the past week. He seems to have some sort of a rash or allergic reaction on his belly, under his leg "pits", and around his groin. I've been giving him Benadryl the past few days which seem to help the itching but it's made him very sleepy. He's been scratching so much lately one of his nipples is so raw it's starting to bleed. How can I make him feel better?
Any suggestions?!