There are three types you can choose from that are Nintendo DS. The original DS is not as widely available. It also plays older Gameboy Advance games.
The one that came out after that is the Nintendo DS Lite, with is lighter (ha!) and has a brighter back light than the original DS. It also plays older Gameboy Advance games if you wish. This version is you can find everywhere.
The very newest is the Nintendo DSi. It has a built in camera (two, actually), and does not play the older Gameboy Advance games. It also comes in a bigger version, the DSi XL. The DSi has a brighter screen as well.
Personally I would go with the DSi (regular size.) It's the very newest, so he'd be getting all of the newest technology, and the regular size is just more appealing to me overall. (and in my opinion, easier on the hands...and I am a long time gamer.)
You didn't mention your son's age, so you may not be interested in this feature, but the DSi will also let you browse the internet (if you are in a wi-fi hotspot, or have a router in your home.)
The DSi should run around $169.99, while you can get the DS Lite for $129.99. Again, even with the price difference, I'd go with the newer DSi.
(we have all three of these in our home.)
This link should give you a size comparison on the DSi and DSi XL. (as well as the features.) The DSi XL runs around $189.99
http://www.nintendodsi.com/meet-dsi.jsp