1)
What is second?
1.
Radak: The second reason I came to you to serve you.
2)
Why is it more proper to serve David's son than David himself?
1.
Malbim: I hope that David will agree to cede kingship to you when he is defeated and understands the end of the matter. Also he will be pleased that I serve his son, and not a stranger! Also you will have a strong love for me, because I was the servant and close friend of your father.