Antihistamines such as Benadryl will only help with allergies and that is not likely in a young puppy. The most common cause of generalized itching in young puppies is dryness from bathing.
If the skin looks normal with no redness, sores or rash it is probably due to dryness. Using a mild puppy shampoo like aloe and oatmeal as well as a “leave on†conditioner can help. The best long term measure is using an omega 3 fatty acid supplement but it can take a few weeks before you see the effects of it.