Porn sites are no more likely to have malware than a lot of other sites. Most anti-virus software does a good job of protecting you against such threats as long as they are regularly updated.

Chances are any attempt to use technical means to prevent access to porn will fail. Your 12 YO kid has more time to learn how to get around those means, then you have to learn how to block him.

I think you need to look at this the same way you would look at him bringing home a "men's" magazine. You can't prevent that either.

Are you willing to subject him to regular humiliating and degrading searches (electronic or physical) to prevent him accessing these kind of materials? If so, feel free to proceed. But know up front you will almost certainly fail, and you need to figure out what you are going to do when you find out just how bad you have failed.