Most Helpful Guy
You tell him he's being an ass. That you are trying to support him and want to support him, but you are going to stay away from his games until he decides whether he wants you there or not. Then every time he loses and acts like a child just walk away, because you don't have to put up with poor losers Then, when he has a bad game don't compliment him because he is going to take it badly, he's not really interested in your opinion of how he played. That's between him and his team.
He should recognize then that you have some boundaries of what is ok and not ok in your relationship. This is a mildly abusive action he is taking and it really is not ok. Once he gets that you are not going to put up with it then you can start deciding what is appropriate to support him, like going to some of his games etc. I'd stay away form the compliments though until you guys get on some understanding of how you are going to work together when he loses a game.
Most Helpful Girl
Well... tell him to make up his mind: either have you at the games or not, but that you won't stand one more rude attitude towards you, cause you have nothing to do with this. You're not a toilet, so if he needs to discharge his frustration he has to do it somewhere else, cause he'll end up losing you.