University of Nebraska also offers a number of online courses.
Most universities have "distance" educational offerings (online or blended). Your in-state institutions may be a good place to begin looking.
Many also have options for non-matriculated students to take a course here and there, provided that the student is otherwise qualified for the course (in your child's case, the high school transcript showing the courses he's completed should suffice), and that the instructor approves. Usually such courses are in a foreign language or in mathematics, but there isn't any reason that I can envision why a high school student should not be able to manage the expectations and environment of a social science course or one in the humanities.
