I think boys are also secretive when they are up to something.
probably it's nothing ... but possibly it is.
as to what you can do .... that's a little harder.
at 14 it could be anything (probably it is just girls)
but if he's anything like I was - he's a bag of unsocial hormones looking for release .... in privacy
I hated talking to adults ... I hated it more when they tried to quiz me .... and I hated it even more when they took no interest in what I was doing.
you know your son ...
if he's trustworthy ... trust him .... if he's getting a little wild ...
insist he only surfs for broken periods of time (controled by you .... and kept random)
you could use an unerase prog (might .... might not work)
a keylogger .....
http://www.actualkeylogger.com/
but remember ... you're going to have to explain what you've done at some point