
If he only gets this for 2 months of the year, it not likely to be the food he eats every day that triggers it (or he would have it all the time).
It could be a hot spot, which if aren't treated quickly, will spread, the dog chewing at it can break the skin, leaving it open to a secondary infection.
Just a thought, have you treated your home for fleas recently? The flea population can bloom with the heating on this time of year, a dog nibbling at a flea bite could trigger a hot spot. A flea- allergy can cause this type of itching too.
But, there are many possible causes, best for a vet to diagnose.