It is my understanding, that only alimony is affected by an affair… Namely, he would forfeit his claim to get alimony from you. But since you and he have already reached an agreement on alimony and there doesn’t seem to be any real assets, there is really nothing, that I can see which you can do. I’d check with your lawyer though.
Further, I’m not sure that webcam sex would qualify as “real sex” in terms of the Criminal Conversation statute. Again, I would check with your lawyer though. Also a heads up, be careful how you collect the “proof”; if you break the law in its collection then it is inadmissible in court.
Hope that helps.