The model depends on what capabilities you want. Video & music or just music. I bought my dd a Nano (in black - she was VERY specific on what color she wanted) and I wasted $$ on buying the 4g model because she will never fill it up. Literally, while the Nano I ordered was being shipped to me, Apple released the video IPOD.
Be sure to look for discounts, like an education discount if you buy direct through Apple. Also, find out what color (usually black or white) your son prefers.
Buy a skin to protect the IPOD and include it as a part of the gift. DD opened her Nano gift first and then the skin and the nano went right into the skin and hasn't been out since. The skin I bought is made of clear polycarbonate and has a window for the dial on the front and access openings everywhere needed.
By the way, I did shell out the extra bucks for the extended warranty - I think it was about $70 for 2 years. We haven't needed yet - but, from what I've read - if you don't have it and something happens to your gadget - you may as well throw it in the garbage.
PS...I also got DD's name engraved on the back of the Nano for free. It's small print and looks nice.