Isn't he the son of Joseph? How is it he says "I came down from heaven"?
Well, you need to think spiritually(spirit) rather than carnally (flesh).
"The words I speak are spirit and truth" said Jesus. His words are therefore the words of the Spirit which dwelt fully in him.So what was it(he) who came down from heaven? He (it) was the Spirit of the Father that DECENDED on the son. The words the son speaks are the words of the Father by the Spirit of the Father. And if the Spirit of the Father is the Father, then it was the Father, by His Spirit that came down from heaven whereby the doctrines Jesus taught were not his own (he says) but the Father's.
Jesus the anointed one, is literally the son of God, his Father and Mary, his mother.
Matthew 17:5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
Luke 3:23 And Jesus himself began to be about thirty years of age, being (as was supposed) the son of Joseph, which was the son of Heli,
Joseph was betrothed to Mary at the time that Mary became pregnant with Jesus Christ by a miracle performed by God in Mary's womb. God created a sperm cell in Mary that impregnated the egg
His conception was an act of God in Mary's womb, and that was God's idea, not man's idea who does not live in heaven, thus the conception is how Jesus came down from heaven. His beginning and his purpose was not conceived by men on earth but by God in heaven.