Never use people shampoos on dogs. They have different ph factors and people shampoos are not good for dogs. Including baby shampoo.
Buy lanolin/aloe/oatmeal based shampoos which are formulated for DOGS and are quite mild. Also, frequent bathing is bad, not good. Boxers have no undercoat and frequent bathing dries their skin more than other breeds.
Tell your vet who suggested baby shampoo he needs to go back to school and start looking for another vet. This vet obviously is lacking in good common sense as well as ideas to solve the dandruff problem.
I agree with Bustermom, this is an indication of a food allergy. You should check into our feeding forum for information on good quality foods which you can choose to feed your dog. Additionally, you can add Omega oils to your dog's food (available at any health food store), which will be of tremendous help in maintaining soft, supple skin on your dog.
Good luck.