Wyspianska:
Ask him instead of random people
wyspi you've beaten me to it!!!!
I could give no better advice than that, a relationship is based on three main features, emotions, trust and communication. Out of these three for this situation i would underline two of them, trust and communication, as these are the two most important to your current situation.
So firstly lets take communication, to communicate with him you need to talk to him, as wypsi said, go and discuss with him what you are happy to discuss with some total randoms on the internet. If you can't do that with him but you can with us then what is the point in having a relationship with him, you may aswell call it quits now.
Seconldy the trust element, do you trust him enough to go onto a website without getting involved with or accepting advances from other women??? if not then you do not trust him, clearly, if you are just having doubts then it goes back to point 1, communication.
So my advice is think about the two elements i have just mentioned and do your own summing up, you are the only one who know's what is best to do as your involved in the relationship from start to finish, you know him. We have only heard about him today and his actions on one particular website.
Good Luck
T