My granddaughter had a reaction to Curly Fries the first time she had them. I would not do them again but my daughter didn't worry about it. She let her eat them whenever they were around. So it seems that it may have been something else that was going on at the same time as the Curly Fries. I would say that he may have reacted to something else. One can develop an allergy at any time to anything. So I would test this theory. Give a little milk. Wait. Give a little pediasure several hours later, see if one or the other causes a reaction at all.
Keep the eppipen and some benadryl close at hand just in case though.