No doubt about it, a PSP given his age.
PSP has better graphics, plays games, plays movies, plays MP3's, has Wifi, upgradeable hard drive and has memory card slot. Pretty much does it all.
DS has an OK assortment of games, kinda fun touch screen, Is Wifi but does not play movies, MP3's or have upgradable hard drive memory card slot.
PSP for around $200, if you look around might get it for $160 or so. Overstock or Dell always has the best deals on these. Every few days they both have 10 - 15% off sales which include the PSP.
DS for around $120, might get it for $110. Not much varience in price.