Because I was a 14 year old boy once, I kinda know what he is going through. It may be a little disturbing for the parent, but what he is doing is just exploring his own sexual desires. I think making him feel bad or shameful for it may do more harm than good later on. He needs to learn some self control and keep things in perspective. I'm not saying you should approve of your child watching pornographic materials, but a lot of 14 year olds find this new thing to do and it is fun and feels good. I think if you make him feel ashamed of it, he will start to get angry and resent you some. I would maybe have his dad or positive male figure in his life tell him what he is doing it natural, but he needs to not be so preoccupied with it and more discrete.

That is just my opinion.