Can my son have his own Apple ID on his macbook and still access our iTunes library under my Apple ID?

Only if he knows your Apple ID and password or if you have setup Home Sharing:

Understanding Home Sharing

iOS- Setting up Home Sharing on your iOS device

iTunes- Setting up Home Sharing on your computer

Setting up Home Sharing for Apple TV (2nd & 3rd generation)

It is much better if he can get access with Home Sharing rather than him logging into your iTunes account.

Peruse Apple's site for information about upcoming iOS8 and MacOS Yosemite releases which are supposed to facilitate media sharing in a family.