Well, the way to sort out for sure what he may be allergic to is allergy testing.
Food is very frequently the cause though, with boxers. And allergies can develop over time too, so it is quite possible for a dog to develop an allergy to something it has been eating without incident for a year or more. Now, it is possible for dogs to be allergic to particular protein sources or to particular vegetables - but by far the most common culprits in dog food are grains. If the food you feed happens to contain any form of corn, wheat, beet pulp you might find it worthwhile trying a different food. Those are the most common allergens, and are also pretty useless nutritionally so nothing to be lost there by trying different food.
Otherwise, I'd discuss allergy testing with your vet. If you can discover what the cause is, then you're on the way to a solution. That sure beats having to use drugs or some alternative treatment for the rest of his life
