What makes you suspect he is viewing your safari logs? I don't know if file sharing users can access those files, as I haven't tried. If you have file sharing enabled and these files can be read by other users the easiest thing I can think of to do is to use a program like SharePoints to limit the directories that users have access to, or simply clear your history.
Safari keeps it's history in the file ~/Library/Safari/History.plist
The other likely scenario is that you are behind a router and he is having the router send logs to his computer. Some routers have the option to dump access logs to a certain ip. If you don't use file sharing then this may be what is happening.