My 6yr old has a Nintendo Ds and he's fine with it, there's a lot of DS games that come with a 3+ rating so he should be fine, plus you can use Gameboys advance cartridges in the DS but it will only work on a single screen not dual screen like the DS.
PS The grown ups love it too, especially the puppy dog game lol
My sons have nintendo DSs and all their younger cousins love them, in fact after seeing my sons some of them have since been bought DSs, including the 5 and 6 years old. As SweetD says you can use Gameboy advance games on them aswell.
right now, the game boy advance has plenty of good games.
Unfortunately, these games were published years ago ( mostly between 2001 and 2005 ), so they might not be available in all shops now. The recent games are relatively poor... and the shops prefer to stock recent games rather than good games.
You will find a few masterpieces if you search.
the DS plays both DS games and game boy advance games, so it is probably a better investment, if the kid doesn't break it.
It would be helpful if you felt confident using a handheld console. If your boy is reading then I'd suggest The ds as there seems to be more variety especially in educational games coming through. The GBA is excellent too though. I have both systems and think they are both good. Although the GBA does have a large back catalogue but may "die out" new software wise soon.
I recommend the Nintendo DS,if he likes dogs get him a Nintendogs pack as it is age 3 and over.My eldest son bought the pack and I was so smitten I bought the Labrador pack for myself at the ripe old age of 44,its great fun!