There are two things here. One is your iTunes library. The other is your iTunes Store account (your Apple ID).
iPods are synced to an iTunes library, not to an iTunes Store account.
However, your computer is authorized for an iTunes Store account. One iTunes Store account can have up to FIVE computers authorized to use it. And one computer can be authorized to use multiple iTunes Store accounts.
So, this is what you can do. On his MacBook Pro, run iTunes and log in using YOUR iTunes Store account (Apple ID). From the menu bar, under Store, select to Authorize This Computer. His MacBook Pro is now one of the (up to) FIVE computers you can have authorized to use your iTunes account.
From the main iTunes Store screen, find the Purchased link over to the right, under QUICK LINKS. On the Purchased screen, click on Music and download all the songs that are "his" songs to his MacBook Pro's iTunes library.
NOTE: Alternately, you can use something like a USB flash drive (or external drive) to transfer all of his song files from your computer to his MacBook Pro, instead of re-downloading from the iTunes Store.
He can now create and log in to his own iTunes Store account on this MacBook Pro. He can authorized it to use his iTunes Store account, so now, his MacBook Pro is authorized for your iTunes Store account AND his account. However, if he logs into his iTunes Store account, he is using his own account to make future purchases. But the old purchased songs (from your account) are still there and should still play.
NOTE: If all of his iTunes Store purchases (using your account) are songs (not videos) and fairly recent, the songs do not have any DRM (copy protection), so it may not matter if his MacBook Pro is authorized for your iTunes Store account or not. Older songs purchases from the iTunes Store do have DRM. You can tell by doing a Get Info on a song and looking at Kind, or by adding the Kind column to an iTunes song listing. If it says Purchased AAC audio file, there is no DRM. If it says Protected AAC audio file, it has DRM. Purchased video content has DRM, but free content (such as video podcasts and iTunes U) does not.
Content with DRM requires authorization (of the computer). But for content without DRM, authorization should not matter.
