I have no experienve with teenagers, but.....If you do confront him, did you have permission to look on his phone? If you didn't, then you shouldn't technically know about it, kwim? So if you do say something to him, make it a general comment/discussion, rather than 'I was looking on your phone and found this' then he will just get defensive and not trust you anymore. I know I would feel violated if my mum was checking out my stuff.
And I agree that it is something that should be spoken about openly, that porn isn't realistic, but that he is a teenager and will most likely not stop looking at porn of some sort...ever!!
Good luck.