If you were using Firefox you could use something like Enhanced history manager to see what has been going on https://addons.mozilla.org/en-US/firefox/addon/420 however from within Safari you already have access to about all you can see.
Your best option of you have concerns would be to use something like Spectorsoft http://www.spectorsoft.com/ as it is a stealth program that remains hidden on the machine but will allow you full access to everything that is done on the machine.
THis software will show you when and where your son went how long he was there and also allow you to print screen shots and access full listings of Instant Messages, Emails, Web Browsing etc.
Let us know if you need more info.
The spector software will work for PC and Mac machines.
Now as to the malicious software, It is possible for a Mac just like a PC to become infected by spyware and malware.
Your best course of action is to periodically check your machine for viri and spyware by using a service like Little Snitch: http://www.obdev.at/products/littlesnitch/index.html
Are you getting a lot of pop ups or any consistent
Let me know
I have never personally come across an erroneous Google search, The only way that I know of to search anything on goggle is to type it into the search box yourself.
Let me ask you if you or anyone else other than your son uses the machine?
If so does anyone else get popups or strange searches opening up on them?
I am sorry to say but it looks like you may have to keep an eye on the history and surfing habits until you can find the root cause.
LEt me know if you need more info.