How will he use it?
The 30 and 60GB ones are really mini computers -- they have actual hard drives and therefore can skip, etc. If he plans on watching video with it you will need one of these. But if he wants it for exercise, or exclusively listening to music I would go with the Nano. The Nano can hold anywhere from 250-1000 songs depending on what size you get, and unlike the Shuffle, you can download multiple play lists (work out music, chilling in the coffee shop music, driving in the car music) at the same time. You can also use the Nano when exercising and it can store still photos.
If he really wants no frills, then the Shuffle would be the way to go, but with that you don't get any sort of navigation screen. You can't see the title of your songs and you can only play them in the order you put them in the folder on your computer or in random shuffle mode. The up side is the price and the fact it is incredibly tiny.