'allo! I'll start with the standard disclaimer: I'm not only not a licensed therapist. I'm actually possibly the worst person to give you advice, because I've been struggling with this general issue for some years now. I offer my perspective in the hope that it will help. I'll count on others who are a little more distant from the situation to offer good advice.
I sure understand your fear. You're a mom, so of course you are trying to help your son in any way you can. You don't want him to go to hell, and that's rule #1 of being a good mom. You're more afraid for his life and soul than you would be even for your own, and -- if you're anything like my mom -- you'd jump off a bridge if you thought it would help. However, jumping off a bridge won't help. Indeed, (like most moms in this difficult situation), you basically have two tools in your toolbox, and neither of them are working. Because of that, you think you've failed. Don't think that. You haven't failed your son. It's just that the tools you're using are the wrong tools for this job.
First, you're trying to block his porn intake. I state categorically that this is impossible. It used to be merely "bloody difficult." In the 1950s, you still had to buy pornography at a store and smuggle it into the house (or "use" it somewhere else), like beer. Of course, if you've ever loved an alcoholic, you know that the inventiveness of the addict is almost limitless, and, even with those obstacles, it is very, very difficult to block their intake by force.
In the modern era of pornography, it is flatly impossible. I'm actually quite impressed that you've detected as much as you have -- although this is aided by the fact that your son is apparently not even taking the bafflingly obvious step of deleting his web history (or using private browsing mode! Or, I'll hazard, deleting his cookies! Good gracious, he's simple!). We have ubiquitous internet access, every filter can either by bypassed or broken (most relatively easily), and you are several decades behind the technological curve relative to him. When you take away devices like his PSP, at the very best you are creating another small obstacle that he will get around in a matter of hours. At the very worst, you are taking away a potentially valuable distraction. (There've been days when the only thing that's saved me is a six hour round of Team Fortress 2. Of course, that's a bit excessive, too, but that's another conversation!) You cannot stop him from getting to pornography by cutting him off from it. More importantly, even if you succeed at somehow cutting it off, you won't touch the underlying disorder. As soon as there's a crack in your defenses -- or he gets old enough to move out -- the disease will be back worse than ever. If change is to come, it has to come from within him.
Second, you're trying to talk to him. You've been doing so for a long time. You're trying to argue, cajole, and generally educate him out of his problem. This is good and helpful to a certain point. He needs the intellectual tools to understand why he ought to stop this behavior. But it's only helpful to that point. Once he knows, he knows, and, unless he has doubts about the teaching, further reinforcement can only make him dread "those talks." Shaming him certainly won't help. Either he knows that what he's doing is shameful, in which case you are only driving him towards despair (a real risk with a habit as pernicious as masturbation), or he doesn't actually believe he's doing anything wrong, which I hope is not what's happening here.
I'm sure it can't help that it's his mom talking to him about his masturbation habit. If there is a single person on Earth with whom a young man does not want to discuss his ejaculatory habits, it is his mother. That's the most uncomfortable thing in the world. Of course, sons aren't eager to talk about sex with their fathers, either, but it is much less painful. Fathers relate to their sons as only men can, and that is vital when talking about a problem that is as intimately masculine as this. I know that women struggle with masturbation and pornography too, but they do not struggle in the same way. They also don't communicate the same way: that caring, open, sharing, maternal concern you have for him is just the opposite of the deep need he is feeling to be reassured without being pried open. It would be ideal for his dad to talk to him. It would be even more ideal for his dad to actually be a role model for him, but it sounds like that's impossible. Well, tough grits. If his dad's a shameless masturbator himself, you're right: any advice from him would be worse than useless. We will have to count on God's grace working in other ways.
And I'm afraid that's what my advice boils down to: count on God's grace. Have one more talk with him. Tell him that you trust him, that you love him, that you believe him when he says that he's trying to break his habit. Tell him that you've only tried to help him in the only ways you knew, but that you don't really have any power over him, especially now that he's an adult, and if he is to change, it can only be by his choice, his strength, and God's grace. Lift the device restrictions -- all of them -- and tell him that his pornography is his responsibility now. You're there to talk with him, shame-free, if he ever wants a friendly word of encouragement or advice.