C.B.
My kids have an older verison of the just plain "DS" that still has a slot that completely fits the Gameboy games. After that came the DSLite which has a slot for Gameboy games but the gameboy cartridge sticks out when you put it in. After that came the DSi (and DSiXL with better internet browsing) which does NOT have a Gameboy slot. Only the DSi & DSiXL can go online.
Between my 2 kids we probably have about $500 invested in 2 DS units and a tons of games (at about $20 each). We decided to go the handheld route instead of the tv game system so they can take them along on rides or for long waits at the dentist, etc. Also, DS has MANY more game releases as a platform than the PSP handheld. They still have their GameBoy Advance SP units and still play those as well. The DS has the dual screen with touchscreen for many games, but the GameBoy is just such a small easy-to-hold format that your son will probably still play that one as well. Any of the geek/gamer clerks at GameStop can give you tons of good advise. Enjoy!