(This is so strange... I started looking through this post b/c of my dd, but so far what I've written has been about me. Maybe b/c she's still several months from being 4 yrs old, so there's just not that much to say about schooling her yet...) ANYWAY, I took the SAT in 7th grade with no kind of afterschooling or formal prep (I think I did look at the types of questions so I'd know what to expect). Did well enough in English, just missed the cut off in Math. Took Creative Writing at CTY that summer -- couldn't have been happier. The next year, I too the SAT again, did fantastic on math. No prep or rhyme or reason for it. Well, here's the reason: you don't need THAT much math to do well on the SAT -- the ceiling is pretty low, course prep-wise. One year was enough to make the difference.
I don't think I or my parents knew about the SET at the time; being more accelerated in math than my school allowed me to be might have made a difference in THAT particular outcome, but that wasn't the point of the talent search for us. For me, going to CTY was a wonderful relief from the monotony and outlier status of regular school. They never expressed it this way, but it was a gift from my parents. I felt no pressure or expectation from them; just the freedom to be myself and be surrounded by peers.
My point is, your DS will be ready when the time comes, and you don't need to do anything to make it so. Five years does seem an awfully long time to be planning for it (says the lady who will probably celebrate her dd's 5th birthday by applying for DYS...;) ) But if it's very important to you, just get him one of those test prep books a few months before. Princeton Review (at least a decade ago!) writes in a particularly accessible style and has good strategies that I think a gifted kid would appreciate.