So, I have a sort of similar situation to yours, but with role reversal.
You can read my thread here: Jealous of wife's masturbation
Long story short I discovered my wife was choosing masturbation over me and flat out lying about it. I was dealing with a lot of rejection when trying to get sex and when I dug deeper I found out she was going solo, with porn all the time. This hurt me. Now the porn is more the less a tool for her to get off quicker and get the job over with so I'm not really upset with the porn as I am the rejection and deception.
From your situation it sounds like you guys have healthy sex life. So first there's a few things you need to ask yourself.
1) Am I satisfied sexually and is he meeting my needs or leaving me high and dry?
2) Is he cheating on me or trying to cheat on me?
3) What bothers me more, the masturbating or the porn itself?
It would seem #1 is ok. It would seem #2 for you is okay. Your feelings on #3 will hold some clues for you to understand why this is bothering you.
As far as a plan of action, here's what you need to do. Don't bring it up to him for awhile and drop the subject. Take some time to reflect on why it bothers you, because chances are he doesn't understand. And if you don't understand clearly you won't be able to make him understand.
Remember, it is scientifically proven that people behave differently when they know or believe they are being watched vs. when they know are believe they aren't being watched. Since he likely doesn't understand your feelings it's likely he'll continue, but he'll just do a better job lying in order to protect your feelings from being hurt and from him having to be bothered with it.
During this time I would check his browser history or his phone. Try to understand his patterns. See what he watched and when. You might get a better understanding of what triggers him to watch porn or what types of porn and you may be surprised. Once you have a better understanding of his habits and your feelings about them you can re-approach the situation later and hopefully have better results.
Remember, these days watching porn is considered very normal. That doesn't make your hurt feelings any less real, but most people will approach this from that standpoint first.
Now I don't care that my wife watches porn and I understand sometimes people just have to masturbate. What hurts me is when she chooses that and either lies to me or ignores my needs. Today is a prime example. My wife is on her period and during that time refuses to fool around with me in any fashion. Totally off limits. But I'm 90% sure she masturbated today. When I check her phone I'm almost certain to find evidence of that. That is unfair, control and lying.
It could be worse, it could be cheating. But I have an unfair sexual partner, and that's not cool. Question is do you have an unfair sexual partner or just a guy who likes porn on his time?