Clearing up a couple of things:
Rice Chex are wheat (and gluten), egg, nut, soy and dairy free. They should be safe.
Quaker Rice Cakes are cross contaminated with peanuts. I'm not sure if you suspect a nut allergy, too, but if your little one has never had nuts, i'd stay away from Quaker brand rice cakes. You might check to see if your local grocer carries another brand.
You shouldn't have to discontinue Zantac for allergy testing (maybe the doc misheard you and thought you said "Zyrtec"??). For skin testing, you need to discontinue anti-histamines (like Zyrtec and Benadryl) prior to testing. Skin testing results are immediate (within 20 minutes). RAST testing is a blood draw test where results are usually available in about a week. You DO NOT have to be off any meds to get a RAST test. RAST testing tests how many IgE antibodies your body has produced to a certain food, not your body's histamine reaction to a food (like the skin test does), so you do not have to be off antihistamines to do the RAST test. Both tests have high false positive rates.
Your child's reaction to egg sounds like a "textbook" allergy. The reactions to dairy and wheat could be a variety of things from an intolerance or an eosinophilic disorder, to a true allergy.
Barley and Rye are not wheat, but they do contain gluten. People with a gluten intolerance cannot have wheat, barley, or rye, and they often avoid oats b/c they are cross contaminated with wheat. Some kids with a wheat allergy (which is different from a gluten intolerance) can tolerate barley and rye. Others (like my son) react to all three grains.
Ok, stepping off my soap box now.

--Katie