abouteve, fb don't have the resources to check EVERY picture posted, they rely on other members to report indecent images & when you sign up you agree not to post indecent pictures. So OPs son has flouted fb rules, and can have his account suspended.
OP, I would either 1) remove pics from fb myself. If no password is forthcoming, then 2) report the images to fb via your own account (if you don't have one set one up for this purpose.
Then I would take the pics off pc, and clear the hard drive, camera/phone (whichever he takes the pics on) would be confiscated. Internet use SEVERELY restricted or stopped altogether for a period of time you feel suitable.
Full account access to fb, msn (you can save convos) , email, other sites etc to be given to YOU & checked by you on a regular basis. Him having knowledge that at any given time you will check on his activity on pc & no unsupervised access. He has to wait until an adult can be in the room for entire session, before he uses it.
No friends over for a specified time, then when they are, not in bedrooms. Then if you relax that, no door closed etc.
Take away everything he has exploited & make him earn it back one by one.