I can't speak for the other systems being mentioned, and my kids have not had a chance to play on an ipad (other than brief moments at the store).
But my DS wanted an iTouch for his birthday. I said no way, too expensive. However, when i started looking at toys. I realized I pay $25-40 for toys that get played with once and then shoved in a corner. He has several extended family members that give him gifts each year. It just made sense to have them all go in and get him what he really wanted and would really use.
I can't say enough about how great the iTouch has been. I ordered an older generation model off the refurbished section on apple.com. I believe it comes with a one year warranty. I wanted it to do three things. Play music, games and movies. I didn't care about being able to facetime or take pics. So I was able to get more memory and still come out cheaper than the lowest priced current model. I believe they are currently offering a 32 gb, 2nd gen for $179. Also, Discover gives 5% cashback.
There are tons of free games, including educational one. Several blu-ray movies that we already owned had digital copies to load on it. There are some revolving, decent digital movies you can download for as low as $5 even though we haven't had a need to do that yet.
I cannot wait for our next road trip!
So basically you would be getting like a minature iPad. If you think size wouldn't matter for him, that is the route I would go.