You know, it might be kind of a radical move, but have you considered calling your son into the room with the computer (without the other kids around) and reviewing what he's been viewing?
Yeah, I know; you really don't want to see it, either, and while I would normally agree this is kind of a "Dad thing", this is one situation where Mom might have a bit more impact.
Let me explain; at 14 years old, the typical young man is rather impressionable and more than a bit insecure about sexual things. He's getting most of his information (more accurately, misinformation) from his peers and the internet, and the odds are, the thoughts of his parents in any kind of sexual mentality - particularly his mother - is almost too much to handle.
So Mom calls him to the computer and tells him to sit down, then brings up the screen with some of the porn. She doesn't go ballistic, which is what he is going to expect, but rather, in a frank and candid manner, discusses far more openly than he is comfortable with, what she sees in these images.
Most porn is about as realistic as a superhero movie, and for most teenaged boys, the very thought of Mom addressing the phoniness of what she is seeing is darn near too much to handle. Imagine, too, Mom very calmly saying something like, "I can't believe you'd really want to masturbate to something like this," as though it were the most normal thing in the world for Mom to talk about to her son. "I mean, look at her, dear. Her acting is so fake it's barely believable. She can't keep her eyes off the camera, and if I made noises like that when your father and I were having sex, you kids would never get any sleep!"
It's a radical move, I know, and it requires exposing yourself to some stuff you find disgusting. It might be worth considering, though, since so far your efforts aren't getting through to him. At the same time, you might lead the conversation over to some web sites where they address some of the problems of porn addiction, including men who would rather sit at the computer and masturbate than have real sex with a real woman. When a guy has become so numbed by the intensity of a fantasy that he cannot function in reality, it's not only a serious problem, it's also extremely sad.
You know yourself and your son better, so it's just a suggestion. If you can handle the conversation in such a frank manner that your son doesn't manage to twist things around and put you on the defensive, though, you might just have a better impact on him in this than his Dad would.
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