Men have a huge curiosity about porn of all kinds.. Something most women find baffling, but there you are. Personally I'd rather be doing it than watching it!
First, is this the first time you think he has been looking? Does he make a habit of it? If he went and had a look once or twice and that was it, then I don't think you have anything to worry about. Check that first. Might even have been some link one of his buddies sent him, saying.,. hey isn'tt hat Fred from billings in this clip? or somesuch bloke nonsense.. If it is a regular thing then perhaps you should broach the subject, but carefully.. it might still be something completely innocent, but men can get very defensive if their sexuality is called into question.
So, first see if it was a one-off, in which case forget it.. second if it is more regular, then you need to talk..