I agree with ppealfruit but think the reply might need explaining more
Look into "histamine rich foods" theres lots of info on the net, but basically, our bodies produce histamine in response to any allergen, ...hence taking ANTIhistamines to reduce the natural allergic response
Some foods contain Histamine, others can be turned into histamine in your body, so your body becomes over loaded & is more likely to respond to true allergens with a more severe histamine response
basically, avoiding foods that top up the bodies histamine, can help reduce the severity of the reaction to anything we are allergic too
for example, tomatoes are a high histamine food, I'm not allergic to tomatoes, but I am allergic to certain mould spores, during the times of year that the mould is a problem, cutting out/down on fresh tomatoes, helps reduce my reaction to the mould.
A friends Sons Asthma was improved by cutting out bananas during high pollen days times, as he is allergic to certain pollens....NOT Bananas :)
Don't be scared off by the food lists, you'll probably find your DS won't react with them all, but keep a mental note of what he eats, & when he gets a flare up, look at what he's eaten in the last 12 hours, that is on the list, & that will be the food you need to cut out during times he is exposed to his true allergens
Otherwise....if no joy with the Head etc at his School, have you considered approaching your local Educational Welfare Officer???.....after all it IS affecting his educational welfare & could affect his attendance ??