What's really important for a newcomer (and even more for a 9 year old) is to be motivating and rewarding. For most kids, modding their favorite game or drawing

What's really important for a newcomer (and even more for a 9 year old) is to be motivating and rewarding. For most kids, modding their favorite game or drawing a few lines on a canvas would be fine. The programming language doesn't matter, but lua for roblox or java for minecraft would be a good fit. Background concepts like how does the CPU work, what's an OS, what's an architecture aren't important for a kid. If he wants to continue programming as a career, he can learn them, but they are not important to get started and will probably lose the motivation.