First, welcome!
Second, some technical notes:
I assume that was the WIAT-III (the IV has not yet been published)? Also, grade equivalents should be interpreted and used with extreme caution, as they have very little relationship to the actual performance, or grade expectations for, students at those nominal grade levels, being essentially statistical transformations of median performance in the standardization group, rather than actual comprehensive measures of performance or expectations.
Obviously, he is very capable, and academically skilled, but I would rely less on the WIAT-III results, and more on his actual performance on university-level coursework, in attempting to place his appropriate academic level.
There are a number of current and past threads, mostly in the Learning Environment forum, I think, regarding radical acceleration, that might be of interest to you. One of your options may be not to do high school in any traditional sense. I take it he had a good experience with the university course he attended? That suggests that he may be an appropriate candidate for simply moving on to more university courses, in-person, or through online, extension, or blended courses, or to nominal homeschooling, using dual enrollment in college courses. (Depending on his holistic development; that will be a question for you and he to contemplate.)
