It sounds like he may have some sort of imbalance; thyroid would be one thing I would rule out for sure. A vet call is definitely something I would be considering, especially if this is something that has not occured before...even if it has, it's NOT normal. It could be something as 'simple' as allergies, or it could be something that will need more intense care, like the thyroid issue I mentioned before. While allergies could be treated with things like hydrocortisone and keeping him out of pastures that seem to bring it on, a thyroid condition may mean medications for the rest of his life, along with a very strict diet.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."