i just got my SP back from my manager whom i lent it to, and i have to say i'm in love with it again. i just got home from work, played a few games in Wario Ware, and i'll try to pick up Boktai where i left it a few months ago. Screw the DS (i've got that too), the SP is still a leader in portables. It's easy to slip in your pocket, fits well in your hand and still looks really good, much better than the DS. Yeah, the DS has a nicer screen and better sound output, but GBA support feels really slapped on, with no way to use the X and Y buttons, no standby mode support, no real extras for GBA games aside from the better screen and audio, which the system has anyways.
So to answer your question, i'd say go with the SP. i'll probably be in the minority since everyone is a whore to the DS's backlit screen, but the system just doesn't have a comfortable feel for GBA games like the SP.
If you do decide to go with the DS, get him a screen protector as well, since the screen will eventually get scratched to hell like his old GBA screen. i've got the Brando and it works very well. It's something like $15 shipped.