On the practical side, I would change your cable settings to block adult content without a passcode.  The same goes for his stepdad's phone.  Many have locking codes.  A serious discussion about what porn is needs to happen between you and him.  He may be looking at it for a variety of reasons, so you need find out his reasons.  I would also want to know how often he's watching.  The good news is that this is an entry into a good discussion at a good age to talk about such things.

i moved my soapbox to the bottom
  

1. Dont be lazy, monitor as much as you can. Why does he have access to renting movies off cable? There are ways to put passwords on such things.

computers -
Use free (x3watch) software (xxx church) that sends you an email if the computer he uses looks at any "stuff". It works really well and no it doesnt harm your computer at all. yes it works on macs and pcs. I think it works on iphone too. Safe eyes is the ultimate software from what i hear on internet censorship, but it isnt free.

The step dad shouldnt leave his phone laying around for the child to get a hold of it. If so the stepdad needs to cancel internet use on his phone. Give up a desire for the sake of the kid. I would question the step dad and the actual use he has for internet on his phone. Its ok if it upsets him to be questioned, you need to know. He is responsible for preventative measures just like you are.

2. You have to teach him why it is wrong to indulge in pornography.

3. Teach him sex is a good desire that he should have.

4. Teach him to think about not letting desires have complete control over what "ought" to be done. Let him know there will be a day for him to have all the sex he wants. WITH HIS WIFE.

5. Find out where he first saw or heard about porn. He may have friends that are showing it to him. You can help their parents as well. If they do not care about it, then you know whose house your child will not be spending time at. period. You as a parent have every right to put boundaries where they need to be to protect your children.

sorry i left out the soapbox -

Porn is addictive to EVERYONE. Fathers allow it in their house because they think they are responsible enough to resist it. Most of the time they are not.

I get sick of hearing people say "Adults and Women are not susceptible to porn". Tell that to the older women who pay monthly subscriptions to porn sites that offer romance and "softporn" as some call it.  Or tell that to all the adults that hang out at work 30 min later so they can get a peak before going home to their families. ( you know who you are)

Lesson is,  society is wrong in allowing pornography to continue.

I dont care how hard it is to get rid of. It is wrong.