The subject is God ("Him"); He is the one who brings many sons to glory. Jesus is “the author of their ["many sons"] salvation through sufferings” – the Father’s plan was for the Son to pass through great suffering to bring about the salvation of many sons.

The cross (suffering) was necessary – Heb 2:17 and 10:4. Unless God had loved the world by sending his only Son to be lifted up as a sacrifice for sin, all humanity would have perished in their sins (John 3:14-16). Ultimately, the love and justice of God that cause the atonement are also the ground of its necessity (MacArthur and Mayhue).