Students under age 13 cannot register for the SAT online, and printable forms are not available online either. They are at high school guidance counselor offices, but I'd prefer not to explain to the high school why my elementary school kid needs a registration form. It turns out that if you call the number listed in http://www.collegeboard.com/prod_downloads/sat/sat-paper-registration-guide.pdf , they will send you the form.

For identification, I plan to use the form at http://professionals.collegeboard.com/profdownload/sat_student_id_form.pdf , which can be certified by the school or a notary public. A small picture is needed. His passport has expired, and the state only issues ID for people age 14 and older.