You got it Deal Grabber... It deals with how much info it will hold.
According to Apple a 8GB will hold:
1,750 songs
10 hours of video
10,000 photos
The 32 GB will hold:
7,000 songs
40 hours of video
40,000 photos
And the 64 GB will hold:
14,000 songs
80 hours of video
90,000 photos
Voice Control
The 8GB also has a few less features, the higher GB ones have voice control.
It all depends on what the iPod will be used for. If he wants them for songs, and you don't think he is going to have over 1700, the the 8GB is an ok choice. But if he wants to download movies, then you might want to get one that is a higher GB. An average movie is just under, or just over 1GB depending on what quality of video is loaded onto it. Plus if your son wants to download other applications, or games, that takes up some space as well.
Hope this helps!