“You are my beloved Son.” The verb in this phrase, translated “are,” is in the present tense and is ontological: it is declaring who Jesus is. The second phrase is God declaring that He is pleased with Jesus, which makes perfect sense because Jesus had prepared himself through his life and now was ready to step into his public ministry.
Some people have tried to say that Jesus somehow “became” the Son when he received holy spirit, but that argument fails on a number of points. Grammatically it fails because to state that Jesus became the Son at his baptism, the text should say, “You have become my Son.” God uses the present tense verb in 1:11, and He uses the present tense verb again at the Transfiguration, when He says, “This is my beloved Son” (Mark 9:7). Both statements are ontological, stating a fact. There is no evidence that either is announcing a change that had occurred.
It also fails because Jesus had been called the “Son” before his baptism, based on his birth and that God was his Father (cp. Matt. 1:20; Luke 1:32, 35; John 1:14). It also fails because what happened at the Baptism was that Jesus received the gift of holy spirit, but there is no other change than that recorded about him. However, Moses, Joshua, and the prophets of the Old Testament all had the gift of holy spirit put upon them, and there is no evidence that then made them “Sons” of God.