My honorary nephew began with my Ruger MkII at very young age (early on, Dad had hands on kid
and gun.)
My most recent outing with that family included kids nine, seven, and five years old who thoroughly enjoyed themselves with the MkII, a S&W Model 63, and .22LR conversions on a 1911 and SIG P226. The Model 63 was a hit because of its size, a problem because the little guys could barely cock it, single action or double. The conversions proved too much a handful for the two younger boys and only just manageable by the eldest. The MkII became a crew served weapon as adults stuffed magazines for kids. Eight shooters blew through 1800+ rounds in about three and a half hours, well over half shot by the three youngsters and most of that through the MkII.
If you buy an autoloader and a
thumbsaver exists for that magazine, buy two or three (be kind to that crew,) you will be glad to have them.
All of my experience with youngsters and handguns has seen the kids delight in revolvers, so long as no autoloaders were available. My young friends preferred struggling with the converted centerfires to the Model 63 (which was its own struggle.)
(FWIW, each preteen had at least one adult, often two or more, within arms reach whenever he had a gun in hand.)
Seriously though, a 22lr would be perfect. A cheap, single action six-shooter would work so he can learn revolvers early on and have no confusion when shooting larger calibers later on down the road ...plus, they can be had for fairly cheap