As I said, I am a prude and I think saving porn on an ipod or any computer is horrible. Suppose his boss or somebody else sees what he's got stored on his ipod?
Tarpon, I tend to agree with your general advice, but I seriously do have to wonder in what possible scenario would one's boss would be looking at one's personal iPod. Like, seriously. I'm having a "if it wasn't for my horse, I wouldn't have spent that year in college" moment trying to fathom how in the hell this is even remotely applicable anywhere, ever.
Anyway, OP, do you have a problem with his saving the porn? Or deep down do you actually really have an issue with porn itself and don't want him looking at it at all? He needs to respect your wishes, and after 3 serious discussions, there are no more "mistakes" or "accidents" if something gets saved, and that's the issue. And threatening you with making a "secret file you'll never find" is just immature and mean-spirited, and definitely an indication of abuse going on.
However. If your problem is actually with him viewing porn, you really do have to come to terms with the fact that this guy is not going to give it up for you (The writing's on the wall, he's never going to stop), ever. If you can't live with a relationship where any kind of porn-viewing is not an option, then this is not the relationship for you, and you just need to accept that and move on now, so you're free to find somebody else that actually can respect that.