Do you know what he wants to do with it? They're vey flexible devices, which can used for a wide range of tasks from coding, automation, or emulating old video games. While some things are required regardless of the intended use, others will vary.
Things you'll need:
MicroSD Card (minimum 8GB, but 32GB is a good cost/size balance).
Case
Power Supply (USB-C for the Pi 4, MicroUSB for the other models)
Keyboard
Monitor or TV (with HDMI socket)
HDMI Cable
If he just wants to play old video games, then I'd recommend the Raspberry Pi 3 Model B+ (the 4 Pi 4 is faster, but runs hotter and the emulation software doesn't fully work on it yet), coupled with a RetroFlag case (they're shaped likle old consoles, such as the Megadrive and SNES, some come with a bundled controller).
If he's looking into anything else, then I'd recommend the Pi 4. There are 3 versions, with varying amounts of RAM installed (typically the 2GB model is sufficient). If he wants to connect external sensors, motors, etc., then you'll need a case which allows access to the various pin headers, camera interfaces. The Pi4 does run hot under load, so I highly recommend a case which includes a heat sink.