Rashi (citing Shocher Tov): He was tending the nursing flock for his father. He was merciful, and first takes out the kids and feeds to them heads of the top grass, which are soft. Afterwards he takes out the mature goats and feeds to them the middle grass, and after he takes out the old goats kids, and they eat the roots. Hashem said, he is proper to tend My nation.

Radak: Alos are the nursing animals. They are called Alos due to their children, which are called Ulim. It says "me'Achar Alos", because the shepherd must be in back of the young animals to lead them slowly - "Alos Yenahel" (Yeshayah 40:11), "veha'Tzon veha'Bakar Alos Alai u'Dfakum Yom Echad [va'Mesu Kol ha'Tzon]" (Bereishis 33:13).

Radak: He took David from tending flock to tend Yisrael.

Malbim: There are two kinds of conduct; this refers to running the country. "Uv'Yisrael Nachalaso" refers to ethical leadership; Yisrael is a greater level than Yakov.