My dd12 took the SAT a few months after her 10th bd, so not that young. She was a 6th grader at the time, though, so she just did it through talent search. At his age and at this point in the year, you'd be registering him directly through the testing company I imagine. The only thing I'd be unsure on is what grade level DYS would be comparing him to b/c I don't think that their admission criterion list those tests for grades below 6th. He'd be considered a 4th grader or what right now? If he meets their requirements for a 6th grader and you have a good portfolio, I'd think that could make for a fairly good application.
Depending on the kid, the ACT might be a better idea. You don't have to do the writing part on the ACT which could be good for a child who isn't fast at writing. There is also no penalty for guessing on the ACT whereas you lose 1/4 of a point for every wrong answer on the SAT. If he's going to freeze up with worry about losing points for guessing, the ACT might also work better. Beyond that, I don't know! Hopefully someone else with more experience having that young of a child take the SAT will chime in.
