Alma the elder recognized what happened to his son, and doubtless knew that his son would recover. He presents his son to the multitude so that there would be a larger witness to what was happening. Alma the younger had attempted to lead the people astray, and the people needed to see the result. There needed to be an equivalent public demonstration of the change.
Thus, many prayed for him, and Alma the younger recovered and stood to speak.
In Alma 36:16 we will learn that Alma the younger was wrapped up in the Spirit for three days and three nights, but here it says only two days and two nights. John W. Welch noted that this difference was probably due to the fact that the priests fasted. The fasting had to begin in the morning of the next day. Hence, there were three days and three nights for Alma the younger, but two days and two nights for the fasting priests.