Confrontation with eldest son

I am currently having an issue with my eldest son Stash. I was doing the laundry and I walked into his room, he didn't notice me. I looked at his computer and he was looking at pornographic cartoon images of prepubescent girls and boys. I was a little shocked at this due to the content.

I have never had the chance to give him a talk and with his father not around I didn't know if I should. I was wondering if I should talk to him on this matter or let him try figure him self out.