Hello my friend. I feel your pain with three sons myself who have become addicted to video games. The reason why it is so difficult to shut the game down immediately is because the players are usually in the middle of a battle of some sort and do not want to walk away. The player needs the extra time to finish the game, especially if they are playing on line with other players. Try giving your son a time limit before he starts the game so that he knows exactly when the time is up, the game is turned off and no questions asked.
When you are telling your son "no" to sleepovers and to having friends over, it is a punishment for something? Make sure that your son knows why he cannot do those activities. Make sure your son is aware of the house rules and the consequences for breaking the house rules before the disobedience occurs. No teen ager likes the element of surprise.
If you need more suggestions, please ask.