Ok there could be a lot of issues here. I would talk to him about all this but I would also try different things with him. Put your son to bed early one night and tell him he has to be home for a special dinner and wear lingerie or serve it naked. Do you know what kind of porn he likes? Tell him your going to make yuor own movie when he gets home. Make him excited to come home. I'm somewhat dealing with the same thing without the porn now anyways, that used to be a factor!!
Then I would tell him it's not error-proof and really tell him how concerned you are with him doing that at work.
If you can't handle the thorns, don't crave the rose!!