en_tn/1sa/17/57.md

in his hand

Here "his" refers to David.

Whose son are you

"Who is your father"

I am the son of your servant Jesse the Bethlehemite

"My father is your servant Jesse, the Bethlehemite"

your servant Jesse

David referred to his father as "your servant" to show that his father was faithful to King Saul.