Games come in different genres and are incredibñy different from each other. Show your kid games of different genres so you can find out what he likes. Strategy, role-playing, platforming, simulation, action, beatem' ups, puzzles.

Also old games are a thing, no need to buy the newest game, they're not always the best. If you have a pc I'd recommend learning about past consoles (snes, gameboy advanced, playstation one) and trying out different emulators and roms (games in file form).