You know that Aspergers kids take things very literal.
So you need to let him know it was the game that was lost, not him personally being a "looser" .
Also that everyone looses every once in a while and that is ok. It is learning experience.
Or that it is just chance (a bit abstract) that no one can control. and this is fine. It is the way it is supposed to be.
Also explain it is not who wins or looses, it is the fun you are having playing the game.
Also in everyday innteractions, talk about and show how you react to mistakes, bad "Luck" and things ourt of your control.
"Oh darn, we were going swimming and it is raining. I am disappointed, but oh, well, maybe tomorrow will be better. What fun thing can we do right now instead? "
"Oh gee this movie is sold out. What options do we have?" "What can we do next time, to make sure we get into the movie we want to see?"
Go to an arcade and when you loose, show a good reaction to not winning. "Gee, I think it will take a lot of practice to ever win this game."
"Hmm, I am going to try this, even though I do not have any idea how to play this. "
Then when he wins, or you win, be a good sport. "Wow, I won, that was all luck. " "You played a good game to."
We always end our game playing with "Good game!". "Thanks for playing."