Neighborhood school is not an option--should have mentioned that. The magnet is going to be better, but not ideal. However, there are other options--in fact, quite a confusing array--but all are going to require major hassle in terms of logistics.
When DS was younger, I felt more unsure about his abilities. However, he's seeming more gifted the older he gets. He plays games designed for 7 and up, including chess (he can beat me, though I suck at it), can read and write (at about a mid-K level--he just turned 4), and is astonishing at fact retention. Still, since our experience with testing DD did not really give the results we expected based on the kid we see, I feel a little wary. I really would not want him to feel inferior because he didn't "get in" to his sister's school.