Whilst I agree with all of the previous posters, insofar as 'there is nothing wrong with watching porn', I kinda think some of you are missing the whole point. Watching porn, whether you are single, dating or in a relationship is absolutely fine until it has an impact on your relationship and if they're not sleeping together because of it then it is an issue, whether that be 'cause OP can't trust him or because he's obsessed with / addicted to watching porn.
Short of talking to him and trying to improve the communication in your relationship, I don't see what else you can do. If the communication isn't good and can't improve, maybe it's not worth continuing with the relationship, for either of your sakes.