Another possibility would be for your son to audit classes at the community college. Presumably someone at the counseling center at the college would be knowledgeable about this, especially because they already have services for gifted high-school students.
You might also contact a senior adult center to see if there are some retirees who would be interested in serving as a mentor -- especially a professional person who pursued a career in your son's area of interest. Many senior centers already have this type of program in place.
Another alternative is to use the Internet to locate a mentor who might challenge your son. If you can identify an interest that he would like to pursue, you could use the website of a professional association in that area to see if they have an outreach program, or you could type a few key words into a search engine and find individuals who are conducting research on the topic. This sort of arrangement is not as personal as face-to-face contact, but written correspondence between a child and an older professional can have benefits for both individuals. Good luck!