I don't know what to say to you hun. For once, I am baffled and unsure what advice to give. If he didn't talk to anyone, flirt, put up pictures, details or try to arrange to meet anyone for sex and you know this for sure then it does sound like he is telling you the truth. You said in his history there was only four-five links which he said he clicked on four of them as he got emails but then tried to delete them. Even if he was only on it for a second it would show up in the history..
I did a little research for you and I just registered on one of those sites under a false name. You can check previous activity on them. I sent a "hello" to one person and it showed up in my previous activity and I got an email with the reply. Did you check those things to see if he had any activity?
"Don't ask a question if you can't handle the answer".