Q: share mobile apps across multiple users of same mac?

I have one mac with two user accounts, for myself and my son, and we each have an iPhone.  I have been syncing both iPhones to iTunes on my user account.  Now I want to sync my son's iPhone to his user account, so he can manage his own Contacts separate from mine. I was able to sync his iPhone to his account, and set up Contacts separately, but in his account I was unable to find any of our share apps.  How can I get apps to appear in iTunes in both accounts?