Fate
You play as a kid.

Oh, and someone please tell the OP that BG is beyond me and I'm middle age. :P

Some jRPGs, perhaps? A lot of them are swords&sorcery, there's exploration, inventory management is very basic, stats and skills are automatically increased, and the stories and characters are mostly appropriate for younger children. Pick a chibi title and let the kid grind to level 99, that should net you a few months of vacation time :) I believe the classics of the genre have been ported ad nauseam, so there's a good chance there's a dozen of them on any console you might own. The situation on PC is a lot more restricted, though - less titles and usually Steam-only.

Trine and Trine 2 are medieval platformers with adjustable difficulty settings. Really nice fairytale quality to both.

Lords of the realm 2

Am I the only one here thinking, 'When I was five I could barely manage picking my nose much less playing a video game.'

Anno 1404, of the "building department"! Easy enough to spend some time placing buildings and managing the economy. Has a sandbox mode that's pretty much " set the options and do what you like"..

* Also, don't be afraid because of UBI, the latest patch removes the DRM. I'm referring to the retail version! :-)

Castle Crashers, is in a medieval world is funny whit an exelent sense of humor and is cooperative you can play it thogether

The first assassins creed?

I can confirm kids into medieval stuff generally love Mount & Blade. The flexible options menu and freeform adventuring will see him taking his first castle in no time.