We have two networking PC's using Microsoft Internet Connection Sharing. We have Norton Internet Security running on the host but would like to monitor / control the access to the internet from the second PC. The second PC belongs to a 15 year who has admin rights to his PC. Any sugguestions? Thank you.

How computer savvy is the teenager? The best solution is to always be in earshot/visual range of the PC with internet access...

I'm guessing you have high-speed internet access. In that case, I recommend purchasing a router (probably cost about $50). A router lets multiple computers connect to the internet and also protects from hackers.
Most also allow you to filter websites and times of day when internet access is allowed. If you choose this, read the manuals and configure it yourself and password protect the settings. It might also be good to change the password with your Internet Service Provider so that the teen could not disconnect from the router and directly connect. (These actions do not imply you distrust the teenager; only that you want to ensure he/she is not online at 3:00 instead of sleeping :-)