I think it depends on your DS-if he's going to be looking for rides and characters, then Disney Quest is going to disappoint him. This is mostly a "hands on" exploration. There are a few rides, but they are all computerized-i.e., you sit in something that doesn't really move-and there's a big screen in front of you that makes it LOOK like you're paddling through a river. There's also little stations where you can draw a Disney Character and color on a computer screen.
Make Your Own Roller Coaster is also there at Disney Quest, but for some reason I can't remember if there is a height limit on that one or not-I'm pretty sure there is becasue you have to be able to fit in the device before riding. If he enjoys a more hands on experience, then he will like the Disney Quest; if not, maybe wait until he's a little older.
When we went last December (Me, Disney Parents, and Disney sisters, aged 18 and 9) my youngest sister enjoyed herself, but got bored pretty quick. Older DisneySister and I designed about 150 rollercoasters and insisted on riding all of them, so she had to wait while we did this and there wasn't much she was interested in, after a while, she started to get bored. However, she isn't really into video or computer games, so it might just be her own taste.
When you get to Downtown Disney, head on over to Disney Quest and take a look at the posters out in front that explain the different things for you to do there. If you think it sounds like something your son would like, go ahead and buy the ticket-or you may decide to wait and try it again in a few years.