1)
Why did he tell the youth to hurry?
1.
Malbim: This was a sign that David must flee quickly. 1
2)
Why is it written ha'Chetzi, and we pronounce it ha'Chitzim?
1.
Radak: It seemed 1 that he shot only one arrow - he said "ha'Lo ha'Chetzi" (verse 37; it is written and pronounced singular), but really, he shot three - "Sheloshah Chitzim Tzidah Oreh" (20).
1
Perhaps he told the youth only to take one, lest someone else come in the time needed to gather all three. Or, amidst great emotion, he was anxious to meet with David immediately. (PF)