I'm sure this is frustrating. Here are my ideas and I hope they help!
1. Talk to him about his sportsmanship at a time when you are NOT playing, or about to play, any games. Tell him how much you enjoy being around him when he is making good choices in his behavior. Tell him the kind of improvement you'd like him to work on in his sporstmanship.
2. When you are about to play a game, go over the rules as a family. Make sure all the players understand that the rules do not change during the middle of the game. It helps if you follow the rules that came with the game w/out exception, because then if a question arises, you simply consult the directions provided by the game maker and your son can't argue with the manufacturer!
3. Before the game begins, talk about how whoever wins should be congratulated for winning by the other players. Also discuss that whoever wins should thank the others for playing the game with them. Set up those expectations before the game and if your son can't agree to make those choices, he shouldn't be allowed to play.
4. If during the play of the game, your son is losing and starts getting a bad attitude, I'd explain to him that he has a choice. He can choose to be respectful toward the other players and continue playing, or he may go to his room and complain all he wants about the game. He may return when he is done complaining, but not a second before. I would NOT back down on this. If he wants to complain, he can do it in his room behind a closed door. It's his choice as to when to come out, but he is not to expose the rest of the family to his nasty attitude. This is a hard thing to enforce because we WANT to be around our children, but sometimes the best way for them to learn is to be separated from those they love until they can act lovingly and respectfully.
Of course you probably already know to praise him when he makes good choices with his sportsmanship. Anytime he does well with his attitude, I'd make a point to tell him how much you enjoy playing games with him when he acts like "such a big boy" or whatever helps him feel special. Also, when he's done a good job, ask him if he will play with you again soon. You should also talk to your son about how he feels when he loses at a game, explain to him that his feelings are okay and understandable, but that there are appropriate ways to express them that still show respect to others. He may also benefit from some role playing so he can practice expressing himself in appropriate ways. I think it's important for kids to learn that their FEELINGS are okay, but they need to be careful how they express their feelings. Best of luck to you!