Hi and welcome. Sorry this is going to be short, but I'm at work and expecting an appointment.
I'm sure you will get some good feedback here. The one thing I have to say at the moment is that rigid and strict when it comes to behaviors that are plain and simple not acceptable in the family is not necessarily bad. The condeming and/or shaming is not good; you obviously won't go there because you know how awful it is, but telling your nephew that certain things he's getting into are not OK is perfectly fine.
Also, this kid is 15, his fixation with porn may just be the natural curiosity that comes with being a teenage boy. He's gonna check it out. I suppose one of the questions I have is what kind of porn. Is it simply naked people? couples? gay porn? lesbian porn? S & M? really demented stuff?
You say your nephew was abused and that you are both in counseling - that's fabulous and helpful. Could you bring this up in a session? If you're apart during counseling, can you talk to his T and have s/he bring it up to him?
Sorry, probably more questions than answers.
If you fall down 10 times, Stand up 11.