If you're looking at a 30 gig item, you're looking at a hard drive based system (the video iPod and the Zune are examples of hard drive based players)-- there are also solid state (eg no moving parts, everything sits flash memory) players -- the iPod Nano is an example of this.
For hard drive based players, the answer is no. For some of the flash memory based players, the answer is "yes" -- although those are mostly older models. The reason is that a new hard disk player with 30 gigs will hold a zillion hours of music . . . (with a very rough rule of thumb at 1 megabyte = 1 minute of music, then 1 gigabyte = 30 hours of music (though this varies by compression).
You can readily see that unless your son wants a recording of every noise ever made, 30 gigabytes is going to more than sufficient.
Memory upgradable MP3 players used to be more common, when they were based on flash memory and that memory was expensive (eg I bought a player from Nike that was 256 Megabytes . . . and you could upgrade to 512 Meg). Now that memory is cheap, the new flash memory players ship with 4 or 8 megs, but their compact form factor means that no upgrade is possible.