Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Hives are raised pink-red areas of different shapes and sizes. They are very itchy. Do the marks on his skin look like this? Is he scratching and uncomfortable? Let s rule out other things that commonly cause allergic reactions. Is he feed anything else besides milk? Is he taking any medications? Have you changed his body soap or the fabric of his clothes? Allergy to cow s milk or soy can also cause a runny nose, wheezing, stridor(a rough sound on inspiration), crying, vomiting, diarrhea, constipation, as well as blood in the stool. Does he have any of these symptoms? To prove the cow s milk is the cause of his hives(if you and your daughter will work together) would be to wait until his hives are gone, you can offer soy milk in the meantime, then give cow s milk and see if hives come back. Then you will know. I do not agree with giving Benadryl to a 5-month-old baby because of the potential for side effects. Instead, use a topical anti-itch medicine such as Pramegel, Prax cream or ointment. These are over-the-counter. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Arnold Zedd