The best way to do this is to use the "manual sync" option, which allows you to manually chose which songs are moved from an iTunes library to an iPod.

If you don't use "manual sync," and choose "automatic sync" instead, you're going to run into complications. The "automatic sync" function works by downloading a computer's entire iTunes library onto an iPod. In doing so, it also erases all songs that were already on an iPod. So, at the end of "automatic sync," the iPod music library will match the iTunes music library of the computer the device was connected to.

It gets even more complicated if your son used "automatic sync" when connected to the family home computer, then later tries to download music from his laptop. If he wants to keep the existing songs on his iPod, he will need to be proactive about setting up "manual sync" when connected to his laptop. Here’s how:

1. When he attempts to download music from his laptop's iTunes library, a warning message will ask whether he would like to "erase and sync" with the new computer. Doing this will effectively wipe out all existing songs on his iPod and create a new library of songs. If he prefers not to do this, he should click "cancel."

2. Next, locate and select his iPod under "devices" in the left column of iTunes. Click the "summary" tab and check the box next to "manage manually." Click "apply." This will turn off "automatic sync" and ensure that he does not lose the existing songs on his iPod.

3. Now, he can add music to his iPod by selecting each song from the laptop iTunes music library and dragging them to the iPod icon in the left column.

Every time your son downloads music from a computer, he will need to use "manual sync" to ensure that he doesn't lose songs that are already on his iPod.