Lilith..

I think that you should talk to him... infact I think that this is a great opportunity to do so (easy for me to say as my son is 7 and thank goodness I haven't had to do it yet. LOL). Doing so now, I think will only instill in him the realization that he can talk to you about anything.

How do you approach it? Well.. certainly not in an accusitory manner, infact not entirely necessary that you even bring up your discovery of his actions on the net. Bring home a Victoria's Secret catalog.. sit on the couch when he is there... and slowly bring up the issue of beautiful women and go from there (or atleast I think that is what I would do).