It does sound like either environmental or food allergies. The key is to make sure that you feed him the same ingredients all the time, including treats. That makes it SO difficult, doesn’t it!?!? 🙂
Getting grain-free treats and organic treats may be the way to go for awhile until spring arrives to see if it is environmental allergies. There are allergens in the winter, so that could be an issue to. More than likely, it is food related.