in internet explorer click on tools > internet options > settings then view files. even if he clears the history these will remain unless he deletes files and cookies.
Assuming IE - Ctrl+H shows the history of sites visited
look in
c:\Documents and Settings\USERNAME\Local Settings\Temporary Internet Files
to see the temp files
(you will need administrator rights and
then in tools | folder Options | view |
tick all 5 "display" options
select the show hidden files
untick the two "hide")
you could always install a keylogger if you dont trust him most are free but dont enable the e-mail option coz this gives the keylogger right to access the internet i dont trust them but try it