[pbjmama]

It could really be anything- the most common allergens are dairy, soy, and wheat. Does he have any other signs of food allergy? Liquid cows milk isn't doing it alone- when you hear about a baby having a milk allergy it is generally an allergy to cows protein so anything with milk in it will cause a reaction. Some people are so sensitive that they can not have cow meat.

Dr. Sears is a great resource, did you read this?
http://www.askdrsears.com/html/4/t041800.asp