What is Paul doing in the attic when his Mom finds him?
The passage in question says: “Then suddenly she [his mother] switched on the light, and saw her son, in his green pajamas, madly surging on the rocking-horse. The blaze of light suddenly lit him up, as he urged the wooden horse, and lit her up, as she stood, blonde, in her dress of pale green and crystal, in the doorway.”
The child is trying desperately to please his mother by choosing who will win the horse race. In the trance he induces through riding the rocking horse, he screams that the winner will be Mirabar, and that proves to be true. The child wanted to prove himself “lucky” to his mother, to provide her with the material things as a husband would. The details describing the mother, “blonde, in her dress of page green and crystal,” suggests the sexual undertones. Both her dress and his pajamas are green, the color of money. Sadly the child dies, however, in his fervor to please her, and she does win the race though loses a son.
Paul is trying to get the winner of the Derby when his mother walks in and turns on the light. He is so intent on getting the name that he loses consciousness after yelling the name "Malabar". His mother doesn't know what he means by the name, but his uncle does. Malabar wins, but Paul doesn't since he dies right after.