You both need to turn OFF the sharing on the iTunes Library and turn ON Home Sharing. It is different.

From there you can connect to your son's library and ask iTunes to show you all the tracks that are on your son's computer that are NOT on your computer.
Select the ones you want to copy from his machine to yours, hit the button and the files are transferred from his to yours.
Done

Details can be found in the iTunes help file or at these two addresses
About Home Sharing: http://support.apple.com/kb/HT3819
Setup and troubleshoot: http://www.apple.com/support/homesharing/

Let us know how you get on

P