If it happens again, you might want to try re-warming the mouse. I had a Colombian Boa that was very picky about temperature. After trying for so long, I would re-heat the mouse, making sure that it was just slightly warmer than body temp all the way through. (happened when I was impatient and didn't allow enough time in the warm water bath) She would then find it acceptable and snatch it right up. Also make sure that the mouse is not too warm. That could also be a reason that he is grabbing then spitting it out. Another thing is if it is too warm the mouse will start to cook, which is not good for the snake.