Take it easy. It isn't the end of the world.
He's discovered porn. Yuck for you both. Hey - he was honest with you. That is the first step. This is fixable.
Don't worry about the dating sites, these are from advertisements littered all over porn pages. Believe him when he said he just clicked on the advertisements to see more images, he's probably not lying.
It's up to you as to how you want to handle this. Some people don't mind, others do. Stop and think about how him viewing images of other naked women makes you feel. If you don't like it....tell him. Ask him what he's going to do about it. Compromise.
Whatever you do, don't back him into a corner. Don't yell, don't scream. Approach him non confrontationally. This is probably embarrassing for him. And I know this is tough - if you can do it, you're a trooper.
I'm not a fan of what porn and what it does to the users mind. People will debate with me, but I feel that porn programs a man to want more and more mental stimulation and will ultimately break down your sex life. It becomes an addiction and can cause you to lose interest in your spouse. Some people can handle it, most can't.
Best of luck to you both.