He is right, you can't control what he does. Also, there is the trust issue. Does he tell you he won't do it then do it behind your back. Sounds like he is looking for a cheap thrill, which he could get bored with, hopefully. But it concerns me that he is also watching porn without your approval. If you were watching it with him as a couple I wouldn't be concerned, but if he is doing this in secret, too. There may be something to worry about. I would suggest you get counseling and at some point he might go with you. And then you have to ask if this is what you want the rest of your life.
Good luck.