A Bank Holiday trip to A & E

On Bank Holiday Monday we had to take The Allergic One to A&E for the first time.  The culprit was a Jusrol Pain Au Chocolat which does not list eggs or dairy in the ingredients and he has eaten with relish many times before without reaction.  Our choice of breakfast was supposed to be a Bank Holiday breakfast treat.  

He had a few mouthfuls and began to scratch his face, he was rubbing his eyes and quite quickly, within a minute or two, his eyes began to puff up, his lips too and he was very upset telling us he couldn't talk properly.  We gave him anti-histamine but seeing as his reaction was so bad, the worst in several years, combined with the fact that he seemed to be having difficulty talking made us think we should take him to A&E to be checked out.  He was given the all-clear and was home a couple of hours later, thank you NHS.  

It is a worry, we avoid dairy and eggs strictly but had until this point been more relaxed about the 'may contain' warnings that food manufacturers use at the side of their ingredients lists.  If this is his reaction to small traces of the allergens then what would his reaction be if he ate larger amounts?  It's confusing as he has been eating these Pain Au Chocolat every few months or so for quite a while.  Has the recipe changed?  It's a great treat, he loves the taste and also enjoys making them.  Sad for me that he will now loose this treat and we have another item of food to add to the 'do not eat' list.  Plus, do we now need to avoid all foods that say 'may contain'?