1)
Which brother commanded him?
1.
Rashi: Eli'av, his oldest brother. Radak - it is a Mitzvah to honor and obey the oldest brother. It was the custom for the younger brother to obey the older. 1
1
Why does he say that it was a custom? He just said that it is a Mitzvah, and so expounds Kesuvos 103a! (PF)
2)
What is the meaning of "Imaltah"?
1.
Rashi: I will escape - I will go for a day and return.
2.
Radak: I will escape the king's service a little, and go to see my brothers.
3)
How did Yehonason justify letting him go?
1.
Malbim (28, 29): (a) It was Zevach Mishpachah. (b) His older brother commanded him. (c) He did not go to eat and drink, rather, to see his brothers.