Hi Julia,
Definitely take your son to an allergist who can do a full skin testing for
foods.  It may not be completely accurate, but it can help you identify
certain foods that are causing problems.
If you're doing an elimination diet, it is important to eliminate all forms
of the ingredient.  This list can help you find some lists with all the
names of a certain ingredient.  Egg is also a common allergy.
Good luck,
Christy
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