Don’t do Minecraft when Minetest is open source. It’s not meant to be installed as vanilla per se tho as mods are for building gameplay and vanilla is just a sandbox/canvas to build Minecraft-like or addjacent games. Especially if you can get him on PC, I could be a hopskip away from creating his own open mods (as opposed to the content farm of underpaid “devs” for Roblox which have wild stories if you look it up).

Veloran is also a good alternative for free software with an adventure MMO aspect that might appeal to the guy.

Being projects without a profit motive & a strong community where the players are the developers is a safer route where you would never expect loot boxes, microtransacitions, dark/addictive patterns because those aren’t fun & money isn’t an incentive. With the source available your son is more than welcome to read it to figure out how games work, contribute ideas/code, & learn to make mods which are all great, real-life skills learned while accidentally gaming & trying to make the game better for yourself & friends—and if he’s not a future coder, there are assets & stories to build, or just playtesting a friend’s mod.