At the moment I would not mention this to your ex (as he seems unable to handle things as an adult) - but what I would do is to seek some counseling sessions for your son... as it seems that your son is confused & hurting and he is using these images as some form of numbness while trying to cope with the changes in his life/family... a professional should be able to help get to the bottom of all this before it gets really messed in side of him.