I have OAS and very severe seasonal allergies as well. Here in VA they routinely check for 42 common allergies... I am allergic to 38 of them. UGH!
OAS is tricky and really depends on what is out at the time. Google it and you can find the list of what tree/grass/weed pollens are like what fruit proteins and you can start to learn what to avoid and when based on his specific allergies. For example, peaches and birch are similar in protein structure so I react to peaches when birch pollen is high. Where the fruit is grown can make a difference, too. A locally grown apple will have the pollen from the local flora on it, so he may be reacting to the pollen on the fruit and not the fruit itself.
You also need to be sure to keep a list of what fruits he is reacting to, when, and how often. I say how often meaning "does he react every time he eats it or just some times". If it is all the time, it may be a true allergy to the food and not just OAS. Take this list with you to the allergist so they can blood test for the specific antigen and you will know if he is actually allergic to the food or if it is just OAS. I used this method and found out I am actually allergic to apples, grapes and all melon (I ended up in the hospital after two bites of watermelon at a picnic!)
Another holistic thing you can try (I have not done any of the ones from other answers) is local honey. One tablespoon per day... it will have all the local pollens in it and your body will start to build up an immunity to them. I don't have time for allergy shots, but the honey has really helped me to lessen the severity of my allergies to the point that I can now enjoy the outdoors in the spring and fall instead of being holed up in my house all day.
Good luck!