What thoughtful questions about teen sexuality! Your openness is healthy and as such, I think your teen will find a healthy outlet for his drives and urges.
Do give him more privacy. For instance, a closed door should tell others that it is appropriate to knock before entering. Do discourage him from visiting the sites on the internet for the many reasons you mentioned, but I wouldn't make it into a big deal because you don't want it to become one of those taboo things that kids do just because they can.
There is enough sexuality in modern media to inform kids of the basics and to satisfy healthy curiosity. Do what you can to help him develop respect for women in a way that counterbalances the many ways in which our culture objectifies the female form.
Continue your "open door" policy and invite him to ask you about anything he feels comfortable asking.
No doubt he will have his own private thoughts that he won't be willing to share with his mom, but it will be nice for him to know that you are there if he needs you.
