Okay. You know in your heart what you know... he's addicted to porn. One in four men are addicted to porn.

You say that the incident (that he disclosed) occurred 3 years ago ...but then you said that 8 months ago was another "slip up."

I'm not judging, but you need to decide if this is something you want to live with the rest of your life. He needs professional help for his addiction. It leads to more and more distress. The Destructive Path of Pornography Addiction

Since it bothers you (as it does me) I would suggest you find someone else to spend your time with for life. He might be able to stay a friend, but obviously his desires lie elsewhere (and he'll need or want you less and less as it progresses and it will cause more and more angst in your relationship.)

No one is perfect but knowing ahead of time what is ahead should be enough for you to make a good decision, imo.