If he wants to build the hardware: buy Arduino

If he wants to build the software: buy Raspberry (since the motherboard is already built)

Those are cheap so you could buy all of them :). I would personally recommend the Raspberry Pi because the default OS (wheezy raspbian) contains lot of resources to learn programming as a beginner. If you go on the Arduino way, buy the starter pack, as it contains a "cookbook" with many projects.