If you are worried about one day possibly having to switch him back out and if you can swing the logistics, I think I'd start by trying him out at your neighborhood school for K and test if you want to but not stress out over it yet or wait until the end of K so he's a little bit older. (note - this is based on what you said re the magnet school's K/1 not being a great program - if it was a great program, better than your neighborhood school - I'd send him there).
We've had early testing go both ways with our kids - ds12, EG, has tested consistently over time (although his early scores did improve slightly at 7). DD10 tested 20 points higher overall on IQ at 5 than she did at 7, and has recently been tested again and the lower scores held up, as well as match where she's at in school. She's still a bright, happy, smart kid - but she doesn't have the scores to get into our district's gifted program, and honestly, she would have faltered had we sent her there.
If you do ultimately find yourself in a place where your ds doesn't qualify for the program your dd is in, we've found it actually helps having our kids in different schools because it gives each of them their own place to shine.
Best wishes,
polarbear
