this thread has probably gone cold but in case not: oral allergy syndrome is real allergy, and the OP should get the kid checked out, at an allergy clinic in a big NHS hospital.
In the progression now commonly observed of allergic reactions, oral allergy syndrome is the step that comes before anaphylaxis. In the particular case here, it usually goes hayfever -> much worse hayfever + itchy lips + itchy eyes on eating apples -> occasional rashes with worsening of lips/mouth & hayfever -> Oral Allergy Syndrome every time apples are eaten -> anaphylaxis.
It may never progress beyond OAS and let's hope for the kid's sake that it doesn't, but OAS should ring BIG warning bells and be treated very seriously.
Also the next step beyond not being able to eat rosaceae fruit is possibly not being able to eat tree nuts, and then after that peanuts: tree nuts are hard enough to avoid, but peanuts are a total nightmare. So avoiding all possible escalations is a good idea!
The reason hayfever and apple allergy (indeed all rosaceae fruits: pears, peaches, plums, apricots, cherries...) go together is NOT that pollens are stored in the peel of fruit and veg (although it is plausible, there wouldn't be a pattern of specific fruits going with hayfever if it were just htat), it is because there are molecules on the outsides of the pollens that are roughly the same shape as the molecules on the outside of cells in the fruit. Your immune system recognises things by shape and just reacts every time it sees that shape (which is why allergies get worse over time).
Sorry if it sounds like I'm pontificating bossily, but I've nearly died of anaphylaxis many times, and as it happens I'm also an immunologist. I've just discovered I've progressed all the way to the dreaded peanut allergy and am pretty pissed off with my stupid immune system.... an argument against intelligent design if ever there was one...