I dont think you should enter him into counseling. If the mom is going to let him continue playing the game then you are counseling for nothing. Reinforce that the game is not real. Boys tend to get into those types of games but usually not at this age. This pictures I am sure are disturbing but some how talk to the child about the pictures and see if he can draw GOOD pic instead of mean ones. I would just use positve reinforcement and not to punishment. Its not his fault its the moms fault for allowing her son to play the game. She just dont see what it could lead to later in life. Your doing the correct thing. Keep it up!
Because your stepson lives with the regular strains you mention in your note, therapy could very well be useful to him. You have already attempted to gain his mother's cooperation in exercising more judgement about the content of the games she permits the children to play, and it sounds like you are not going to make any headway in that regard. It is unfortunate that children become the victims of their parent's bad judgement and resentments, and this is ceratinly the case with your stepson. The chronic situation of 'living in two worlds' would be the focus of therapy.
Good. Therapy in a situation like your son's is a useful support for a difficult situation.
Thank you very much for your advice and I will begin researching therapists in our area. My stepson truly is living in two polar opposite worlds. He is with us most of the time (my husband is his primary custodian) and has visitations with his mother, so hopefully this will help swing the balance of parental influence in our favor through his formative years.