where could I get the free keylogger?

There are plenty of cheap keyloggers and filters for monitoring and controlling children access to the net but I would not trust free keyloggers as the chance is too great that it will be malware and will be logging not only junior internet surfing but your banks/credits cards passwords.

Fair warning if your son is computer aware or have friends who are such programs can be bypass if he had any idea you are doing such monitoring.

Booting from a Linux CD/DVD to do internet browsing for example.

Yeah, I agree with Billrm. Cheaper buyer takes the bad meat. Free keylogger may not functional or safe. It's better to choose a safe and useful keylogger to protect your teen, and many keyloggers have free trials, so you can try their free trials first before you make a decision.