Different talent searches handle it differently. As of this past spring, if you use NUMATS, it is a single step process. You input the info online at the NUMATS site after paying the SAT plus NUMATS fees. No paper registration required. If you are homeschooling, you can probably list him as a 6th grader; otherwise their system may not allow you to register online.
Another option is JHU-CTY but they don't routinely test students below 7th grade. If your child is not at least a 7th grader, you will need to request paper registration from JHU-CTY in writing. I think that it may be easier to get an exception approved if you are part of a talent search.
As part of your request, it may be helpful if you can demonstrate that your child is ready to take the SAT independently. For example, he already hit the ceiling on Explore but needs the SAT for entry into a particular program. He is able to and intends to complete not just the 3-4 math sections but the other 6-7 sections as well so won't be a distraction to other test-takers. He doesn't require special accommodations and is able to navigate the building independently during check-in and breaks, etc.
