I would say Garry's Mod + Lua Scripting / WireMod
Easy to get into since its rather old at this point and its easy to get bootstrapped with simple logic in WireMod, Its how I learned programming, has multiplayer and really supports messing around.
Easy to get into since its rather old at this point and its easy to get bootstrapped with simple logic in WireMod, Its how I learned programming, has multiplayer and really supports messing around.
I think since the kid is interested in building a game that they designed themselves, it's better to handover a (simple) game engine to him. A game engine enables way more possibilities than modding. You can add multiplayer over websocket/webrtc, you can write your own shader, you can write your game logic however you want etc. All of that without the overhead/limitation of a modding environment.
Also, out in the wild the job opportunity for Garry's Mod developers is very scarce, if any. Just something to think about if op wants to do some carrier development for the kid while having fun.
My previous comment was more like a personal regret on my self that I failed to convert my modding experience into actual skill in game design/programming. I started it too late for me to relearn all the math and etc. I imagine my career will be a lot different if I started with the intention of designing an actual game.
Yeah, that's all :)