When you tried him in it, was the seat installed, or sitting on the floor/ground? Kids often lose some room once seats are installed.
I agree that you need to try it in your vehicle if you have not already.
I think that this is even more true than usual with the Nautilus. A lot of kids seem to be losing about a slot when the seat is installed in a vehicle compared to when it was tried out in a store (or wherever).
When I tried my DS in Monzogary's uninstalled Nautilus a few days before he turned 7, his shoulders were actually below the second from bottom (!) slots. When we purchased our own Nautilus a week later and installed it, his shoulders were about 1" below the second from top slots. So he lost a slot. (I know he didn't grow that much in a week.
)
As to whether to buy the seat for your son (if he fits in it), if you have the money and realize that he's not going to last terribly long with the harness, I don't see any reason
not to buy it. My DS loves his Nautilus and actually prefers it to his boosters and even his Regent.
He drives my nuts, he is in a turbobooster but every time I turn and look the stupid seat is leaning to the side or he is leaning over or something.
Have you tried locking the seat belt when he sits in the booster? You should be able to do this in your Ford by slowly pulling the seat belt all the way out, then buckling it. I often recommend this for the newly boostered or for extra-wiggly kids as a way to help train them to sit still.