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.
That's an excellent point, my mum read my diary when i was about fourteen-fifteen and it shattered my trust in her, i believe it's why we had such a tough relationship before i moved out of home. Had i not known it wouldn't have made a difference. So maybe avoid telling him you saw it and/or know about it and see if you can get your DH to discuss with him.
However, i do agree that it will be an entirely normal part of him growing up and while you may not agree with it it's likely to happen for a long time. I do think a discussion about how the objectification of women in magazines is not cool and how the porn industry is not realistic and can be considered degrgrading would be healthy in him establishing a more appropriate view of women for when he does become sexually active/in a relationship.

Maybe remind him that seeing as you're footing the bill for his mobile he has to play by your rules until he has a job to pay for his own?