Speaker preferences are such a subjective thing, I would want to take him to the local shops and have him listen to as many types and brands of speakers as the two of you can stand, with music with which he is familiar, and of all of the types of music to which he listens (making a CDR for this is a good idea), and decide based on what he likes.
At your price point, there will be major compromises, and you don't want to compromise on the wrong thing.
At his age, I wanted a lot of bass, but by the time I got to college, I wanted something more refined and was willing to sacrifice some bass for it. (Because I did not have the money to just get what I really wanted, which would have been refined, with deep bass, and would have cost a fortune.) His priorities may well change between now and then.
At your price, I would want to look at used speakers, as you can sometimes get something much better for your money that way. It is more trouble though, as one must be patient and keep looking at local ads (like Craigslist) and then research models advertised if one is unfamiliar with them. That is time-consuming, but it can get you something better than you are likely to find new at your price point.
