Karen,
Your son is probably allergic to something else as well.  Try reading
Doris Rapp M.D., book on discovering allergies.  "Is this your child?
Discovering and treating unrecognized allergies in children and adults".
This book and the multiple elimination diet helped me discover my
children's allergies.  You eliminate all allergy producing foods until
symptoms gone, then add them back in one day at a time and see what
produces symptoms.  What an eye opener.
Janine