It could be casued by any number of things really.
Plants, seeds, bedding, hay etc.
Just make sure that any reaction he has isnt affecting his breathing, and a good wash in a medicated shampoo (like quit-itch) will help with any discomfort.
Finally try to locate whats causing it. If its somethng that can be removed then do so. If not restrict his access to it.
Hope that helps