OP, firstly, really sorry that this has happened, it must have made you feel ill to see that.
Paying for a webcam means that they are intimately interacting with another woman, right? Please someone correct me if I am wrong, but when you pay for webcam, doesn't it switch your webcam on and so the performer and the client can see each other as well as hear each other? Is this not cheating?
How would you feel if your OH was in contact with a regular prostitute, but instead of meeting up with them in person was sending them explicit texts, pictures, sexual Skyping, phone sex, but hadn't actually touched each other? Would you think he has been unfaithful then? I would, and what he has done is no different. He's cheated and that would be the end of it for me. Would NOT be waiting around to see if he would cheat further by DTD. Where do you draw the line then?
Porn - being an observer of sexual acts (usually via some form of tech medium e.g tv, pc etc.) As soon as you start interacting with the sex worker sexually it stops being porn and in this case is cheating IMHO.
I would not give it long until he actually DTD in person however having said that I would urge you to focus on what he actually has done; he is taking absolute advantage of your liberalness around porn, has spent quite a lot of family money and was initially ignoring your concerns and feelings.
Also, if he doesn't see that anything is wrong (which was his initial reaction before he went into bunker down mode) he is unlikely to change his behaviour, and what kind of marriage is it to be sitting there checking all accounts? There are loads of ways to get around it anyway.
Also, what AF said. He could have chosen to bring this up with you and work on it with you before interacting with a sex worker. Now you feel you need to compete with these random women online so that your husband doesn't continue to cheat on you. Your DH is doing alright out of this arrangement OP.
Whatever you decide to do good luck and I hope it works out best for you.