I agree with the first two replies. I would like to add that this is his addiction, and it's not based on anything that you did or did not do.
He sounds like a great person in so many ways. He really needs to get help with this one way or another. If he can't stop, it is an addiction.
Addictions make people lose respect for themselves. If he doesn't turn away from it, it will be very distructive to him. p 0 r n eventually interfers with normal relationships, sometimes, causing a person to not be able to have normal sex at all.
You are right to be concerned. If he could see it for the distructive thing it is he wouldn't want to do it. It could ruin your family.