How do i find out what my 3 month old is allergic too?

See an allergist: You don't tell me what symptoms your son has nor whether he is breast fed. Food allergy is the commonest at that age is food allergy, less frequently there is allergy to pets or mite. To find out, a full history, examination and allergy blood tests +/- skin tests need to be performed. Depending on the history, a special formula may be tried. See a board-certified allergist/immunologist.