Microsoft Flight Simulator 2020 (MSFS) is pretty good, obviously not 100% accurate and some of the physics engine is still kind of mushy but leaps and bounds ahead of Flight Simulator X

Mostly GA-focused, a few airliners and business jets, I think there was a promotion for Top Gun 2 that let you fly a F/A-18E, no combat per se but I'm sure there are third party mods

Detail is as much as you want to dial in; you can plug and play for a little kid or you can do your own w&b, limited fuel, multiple system failures in sideways freezing rain on an NDB approach to a 1000' runway at night

Uses Bing Maps for close to 1:1 world and weather, hopefully your PC is up for it and you have a good internet data plan...

Heard good things about X-Plane 11/12 but never tried it

When he gets older you could also look into DCS World, which is more military-oriented and starts out in a P-51 or a Su-25 and you can plan your own strikes in Georgia, controls are somewhat difficult to program however but tons of add-on modules (WWII and OEF campaigns, aircraft from different eras, maps, etc.)