Jesus was God’s son, because he came into existence by the power of God causing Mary to conceive him, thus making God his father. Jesus had no human father.
Matthew 1:18-20
Now the birth of Jesus Christ took place in this way. When his mother Mary had been betrothed to Joseph, before they came together she was found to be with child from the Holy Spirit. (19) And her husband Joseph, being a just man and unwilling to put her to shame, resolved to divorce her quietly. (20) But as he considered these things, behold, an angel of the Lord appeared to him in a dream, saying, “Joseph, son of David, do not fear to take Mary as your wife, for that which is conceived in her is from the Holy Spirit.
Luke 1:30-35
And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. (31) And behold, you will conceive in your womb and bear a son, and you shall call his name Jesus. (32) He will be great and will be called the Son of the Most High. And the Lord God will give to him the throne of his father David, (33) and he will reign over the house of Jacob forever, and of his kingdom there will be no end.”
And Mary said to the angel, “How will this be, since I am a virgin?”
And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy–the Son of God.
So Jesus, being Mary’s son, was mortal and faced all the temptations to sin which we face, although because he was also God’s son, had the strength of character and love for his Father, to overcome them.
Psalm 80:17
But let your hand be on the man of your right hand, the son of man whom you have made strong for yourself!
Hebrews 1:3-5
He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high, (4) having become as much superior to angels as the name he has inherited is more excellent than theirs. (5) For to which of the angels did God ever say, “You are my Son, today I have begotten you”? Or again, “I will be to him a father, and he shall be to me a son“?