C.,
I believe that too much video games are as bad as too much tv, especially the violent programs.
Your son may just be trying out a new way of expressing himself, but may be taking his ideas from these violent games. You need to make sure he understands that this behavior is not appropriate. You may need to start taking away privileges, toys, or planned outings until he learns that he cannot lash out in your house. You will also need to show him what is an appropriate way to express his frustration and anger.
You should also talk with his father, and you may want to talk with the 11-year-old's mother as well and see how the older child expresses himself and how she feels about it.
Sit down and talk with your ex- when you are both calm, don't accuse him of bad parenting, but talk about the changes you have seen in your little boy. Express your concern that these changes have come about because of the video games. Ask him if he has noticed the same changes when your son is with him. If your ex doubts that the video games are causing the behavior then ask him if he is willing to try an experiment; no video games for a month and see if your son's behavior starts to shift back to his normal patterns.
If your ex refuses to work with you, then I think it may be time to speak with someone who can help you change the visitation rules. Explain your problems, document every incident your son has gone through (maybe even with video tape). Keep a calendar and track his behavior; does it change right after he is with his father and get better after he's been with you a while? These are all things the court will need to know and you will have to prove your case completely if this is the route you choose.
I don't know if you can call child protective services or if you have to go through the legal system, but I would look into all your options, talk with your lawyer and discuss the pros and cons of each option.
Good luck!
Oh, one more thing, you might want to look into a counselor for your son. He might need to talk with someone right now, and the counselor also may be able to suggest other ways to help control your little boy's behavior.