I know that you can have a skin test done for strawberries. If he is allergic his throat/mouth can be itchy or swell up, other symptoms can be signs of a sensitivity. I get hives if I eat too many strawberries, I have it down that I can eat FOUR strawberries within 6 hours, more then that I will break out with hives all over my legs/thighs.
Taken from http://usehealthguide.com/strawberry-side-effect
"Allergy to strawberries is characterized by a complex of symptoms in the mouth, known as OAS, or Oral Allergy Syndromes. This complex of symptoms typically consists of allergic reactions to the throat and mouth, such as itching, swelling and paresthesia to the oral cavity (including gums, tongue, palate, inside of the cheeks) as well as lips. Urticaria, a common symptom in Oral Allergy Syndrome. Other allergic reactions include lacrimation, swelling and reddening, runny nose, inability to breathe through nose, itching and sneezing. In rare cases some subjects may develop more severe reactions such as urticaria, dermatitis and itching, asthma, rhinitis, nausea, vomiting, diarrhoea and cramps.
According to a research people allergic to strawberries can develop other food allergies such as kiwi or pineapples allergy but it’s not a rule and it doesn’t necessarily mean you’ll have to develop another kind of allergy since you have allergy to strawberries. In very rare cases it could end with anaphylactic reactions and asphyxia. What is the solution to this problem? The best treatment is prevention. If you know you have some allergic reactions to strawberries avoid eating any form of strawberries, including raw berries, jam and naturally-flavoured products.
Remember also that cooking strawberries does NOT reduce or remove allergic reactions!"
Best thing to do is limit or take ALL things with strawberries or flavoring in them out of his diet if he is allergic... and you can be tested for it because it is a common sensativity and allergy.