where there's a will.. there's a way. my dad caught me looking at porn.. at first he yelled at me, but did that stop me? nope.. so he put on a program to monitor and block sites, but i figured out a way to disable the program.. then he locked the computer down and put a password on the bios.. so i popped out the cmos memory battery.
so what could he have done to prevent me from looking at porn? absolutely nothing.. it's inevitable that your son will look at porn.. and eventually he'll have sex. you can try and take measures to make it harder for him, but i wouldn't expect it to stop him.. what you should do is talk to him about it. let him know that what he's feeling is normal and that there's a right and wrong way to look at women. your job is to provide for ur kid and to guide him as he grows up, and not control how he grows up.. you've gotta teach him about life and not avoid the topics that may make you uncomfortable. the more open your relationship is with him, the more influence you'll have in his life.
that doesn't mean that you should let him look at porn freely though.. you've still gotta impose some authority.