Blonde - the things you describe about your boyfriend suggest to me that he is unable to manage his problem with porn. If after confronting him about the issue he is still using it and is masturbating to it while you sex life has diminished, he is out of control with his behavior.
He probably wants to stop but doesn't know how to. He has probably acted on his impulse to view porn even knowing that it hurts you. That is quintessential unmanageability and a sign that he has a problem that he cannot control. Additionally, the fact that you have confronted him about it and he tries to minimize the problem by saying it's not a big deal tells me he's in denial regarding the problem. If it's a problem for you, it's a big deal... period. He needs to take you seriously.
Lastly, I would recommend a book to you called Out of the Shadows by Patrick Carnes. You can find it at Barnes & Noble. It might help if you read it first to see if the things described in the book characterize your boyfriend. You may even purchase it and encourage him to read it for himself.
The important thing to realize here is that while relationships are difficult and both parties can contribute to problems in the relationship, this kind of an issue really has nothing to do with you. Odds are that he has brought this problem into the relationship and has used porn as coping mechanism for more deep-seeded problems and pain. It's important to know that you are not the source of this problem.
I hope this helps.