Think about when, and how much, he will use it. My daughter has a DS, and got it at about 5 or 6. She really likes it, but the games are kinda pricey.
She now has a hand-me-down iphone, which since it's not connected, is basically the same as an ipod touch. She gets lots more use out of it. If you spend any amount of time in the car or traveling, it could be a better investment. It may cost a little more upfront, but the games and such are a lot cheaper. Almost all of hers were FREE downloads. She has a couple that were maybe $1 or $2. And one (Little Pet Shop) that was $5. We have loaded it with music, books & tv shows. So when we're in the car for long rides, she can watch an episode, read a book, listen to music, or play games. Lots more options.
She even likes to use the weather app in the morning to see what the weather is going to be LOL