Corn Allergy?

My son (almost 11) has been having stomach pain and vomiting off and on this last week. I noticed every time he would feel sick it was after eating some form of corn. A few years back we did allergy testing for him and corn was on the list of allergies. The doctor and I discussed this and we both felt it must have been a false positive test because he ate corn products often and it didn't effect him, that I knew of. But now I think it is effecting him. He has had stomach aches, vomiting, headaches, dark circles under eyes and asthma for about 2 years. He also had a lot of ear infections before he had tubes placed, and he had his tonsils and adenoids removed last year. I wonder if these are all related to corn allergies? I did a bit of internet investigating, there is a huge list of items that may contain corn. Great! I will start keeping a food diary for him and see what happens. Has anyone ever heard of a corn allegy before?