I don't think there's ever any real doubt that Ponyboy will participate in the rumble. Unlike Randy, who makes up his mind beforehand that the result will not serve any real purpose, Pony decides that he must join his brothers even though he is not really physically fit to participate. He is worried, though.
"I got an awful feeling that something's gonna happen."
Pony seems somewhat relieved when Cherry assures him that no weapons will be used by the Socs. When Darry suggests that he may not be up to the rumble, Pony defends himself.
I've got to be in it. Right then the most important thing in my life was helping us whip the Socs.
Pony promises to "get a hold of a little one," and Darry relents. Pony's belief is that winning the rumble will somehow even the score after the repeated Soc attacks on the greasers; the Socs promise to stay off the greasers' turf if they lose the fight, and Pony believes things will be safer for him and his friends. He also thinks it will raise Johnny's spirits.
However, his premonition of something "awful happening" at the rumble is misleading: The awful things occur afterward, when Johnny dies and when Dally is shot to death by the police.