Here the words "we" and "us" refer to John and all believers. The words "him" and "his" could refer to God the Father or to Jesus. (See: [[rc://en/ta/man/translate/figs-inclusive]])
John was an elderly man and their leader. He used this expression to show his love for them. AT: "My dear children in Christ" or "You who are as dear to me as my own children" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The word "advocate" here refers to Jesus. AT: "we have Jesus Christ, the one who is righteous, who speaks to the Father and asks him to forgive us" (See: [[rc://en/ta/man/translate/figs-explicit]])
"God is no longer angry with us, because Jesus sacrificed his own life for our sins" or "Jesus is the one who sacrificed his life for our sins, so God is no longer angry with us for our sins"