This could be translated with a new sentence: "his mother's son. Joseph said."
## Is this your youngest brother... ##
Possible meanings are 1) "It is a rhetorical question with the alternate translation: 'So this is your youngest brother...'." 2) "Joseph sought confirmation that the man was indeed Benjamin, his brother."
## God be gracious ##
"May God be gracious"
## my son ##
This is a friendly way one man speaks to another man of lower rank.