Flatpack: Fix the Future is the first game in the smash Hyperoptimistic Postapocalyptic genre of gaming. Flatpack is a game about building a new society using pre-apocalypse technology that you don’t always trust or understand, and solving terrible problems creatively.
Maybe a modified twilight 2000 or Mutant Year Zero? It depends on how grim or heroic he wants the game to be.
My guess is he wants a steampunk setting with survival and base building mechanics.
Hyperborea if you like a AD&D style game.
Worlds without Number is similar, slightly more modern system.
The out of print modiphius 2d20 Conan could also interest you.
Dramasystem?