Being "in" someone represents having a close relationship with him, that is being united to him or belonging to him. The phrase "him who is true" refers to the true God, and the phrase "in his Son Jesus Christ" explains how we are in him who is true. Alternate translation: "we are united to him who is true by being united to his Son Jesus Christ" (See: [[rc://en/ta/man/translate/figs-metaphor]])
He is called "eternal life" becasue he gives us eternal life. Alternate translation: "and the one who gives eternal life" (See: [[rc://en/ta/man/translate/figs-metonymy]])