Does sound like he's been exposed to more adult material than is healthy for him. I think you may have to talk to him about sex, explain that the images he's seeing are just fantasy as a lot of porn is degrading but he's not old enough to understand the difference between the pictures and sex within a loving relationship. Focus on the relationship side of things. Obviously not much you can do to stop him from being shown it but try to put it into context for him so that he can have a healthier attitude about it.
If you believe that he has been interfered with in any way then maybe it would be best to call the RSPCC or parentline and discuss it with them for there opinion on the matter and advice on how to deal with it.