I would like to help you with your question.
There are a number of excellent career tests that might be good for your son to take. I am wondering what services are available in the guidance office of your son's current school. These departments traditionally have access to interest career tests, along with resource books.
I would recommend that you talk to either the teachers at his current school to see what is available for testing..or the guidance teachers at the high school he will attend.
I understand your desire to not have him overwhelmed with high school so that he loses interest in college. With so many choices...it is going to be difficult to decide which is absolutely best.
In my experience with gifted children, it is important for your son to feel comfortable in whatever academic setting he is in, and to feel emotionally safe. I would recommend that he visit each school, spend a day in class shadowing another student, and that you also drop by randomly to get a sense of the environment.
Another important factor would be to think about what occupations/careers are going to be relevant at the time he would graduate from college. You also need to have an eye towards graduate school because of his giftedness.
I see that you are offline right now. When you come on line I will be notified. I would be happy to chat.