The line was crossed with sex chat and sharing naked photos with a real person. That is essentially cheating.

With the Porn, you have to consider what your own opinion of it is, as no one here can really give a final answer as to whether it's right or wrong in your relationship to allow it. And those views will vary, but it's an individual choice/value. I will only say that regardless of what each individual thinks of the behavior itself, it is different from actually interacting with another person - whether in chat, video, or real life. porn lacks the interaction with a real live person on the other end so I don't believe it's the same as cheating.

As for his past I wouldn't worry about it, it's in the past and they were young. Doesn't mean they do that or would do that now.