Sounds to me like it could still be the sun. It tends to be much harsher
in the spring and especially
the summer. Every summer when I go to
the beach, the tops of my ears are always the first thing to burn if I don't use sunscreen, and I get horrible peeling and itchy blisters, but I never have this problem when I am outside during the winter. Even if there is another underlying problem with your son's ears, using sunscreen will ensure that the sun does not exacerbate the problem.