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A few ideas:

1) Look at your State's high school graduation requirements. You may find this by reading your State's legislation and/or your State Department of Education website.

2) Look at the websites of several colleges which your child might wish to apply to, and take note of their admission requirements.

3) Here is a post on a recent thread, the second half of which lists some resources which may be of interest.

4) The Davidson Database has helpful articles including Thoughts About College Admission and Writing Transcripts, by Wes Beach.

5) These books by Elizabeth Wissner Gross were written a decade ago however much of the information still applies:
What High Schools Don't Tell You and Other Parents Don't Want You to Know,
What Colleges Don't Tell You, and Other Parents Don't Want You to Know.
These books provide tips and ideas on what to do during the middle school years and high school years to help your child find and pursue opportunities in his/her areas of strength.

6) Because you mention that your child is profoundly gifted and in the 99th percentile, have you considered applying for him to become part of the Davidson Young Scholars (DYS) program?

7) In the event that your username is a real, identifiable name, you may wish to change it to a nickname to protect your child's anonymity, consistent with the forum rules.