In all honesty, is he looking at porn, or just masturbating? If it's the latter, and doesn't involve porn, it's not something you need to get all worked up over.
At the same time, and I'll put this as delicately as possible, how is the "intimate" relationship between the two of you? Research shows that for most men under the age of 60, after about 3 days of no "intimate" contact, it gets more and more difficult to avoid trying to find a... release. This is the point, again, for most men, when the short skirts/tight tops/tabloid bikini images can begin to become a distraction.
Now, all of this depends on your feelings of masturbation within a marriage, something the church is silent on, although others might disagree. (And I'll gladly debate why we feel it's ok. :>)