Use Multimedia Fusion or Construct. The former is what I started with, and the latter is a modern iteration of that. Alternatively, Game Maker. If he hasn't been interested in computers before now, don't ruin the momentum, start from quick and easy, then introduce Lua scripting and if he's still interested, consider Unity or Unreal.
Multimedia Fusion is now Clickteam Fusion. They have a free version you can try. I will say though, if you're not running Windows you're going to have a bad time with it. Their "Mac" version is a 32-bit Wine set up running the Windows version and doesn't work at all on anything modern.
