The YAS-23 is nothing to sneeze at.   If your budget is around $500, a good used one should definitely be considered.  There are some alternatives in that price range, but it's hard to say whether or not any of them are better.  A lot of that depends on the player's goals and how they individually define "better."  But they would come with their own compromises.  If your son wants to beef up his sound after playing saxophone for a while, a better mouthpiece would make more difference than a better saxophone.  And more experience playing would give your son more time to think about what he really wants from a saxophone without being under the gun to purchase one because of an academic schedule.  When you're in the market for a used saxophone, time is your friend.

Some other horns you might consider are Japanese Vitos, pre-Japanese Vitos, Martin Indianas, and Martin Imperials,  The Vitos have modern keywork.  Japanese Vitos were made by Yamaha and Yanagisawa, so they're good quality horns.  The Martins have older style keywork, which might be a deal breaker for some, but they have a rich, sweet, "classic" sound.  

This site has an article with some good tips on shopping for used saxophones.