It is difficult to use human language to describe the relationship between Jesus and God the Father without using metaphor. To describe it as a "biological" relationship seems misleading, since biological relationships are between created beings. If the word is used to convey the idea that the very life of the Father is fully expressed in his Son, it may be helpful.
I think the author of Hebrews uses "Son" to convey the idea that Jesus shares the nature of God the Father, unlike any created being. Perhaps we can say that the relationship between a human father and son is a metaphor of the relationship between God the Father and God the Son, rather than vice versa?
As for the weight of the Son's words in comparison with those of the prophets, I think the difference lies not so much in what they said, but that Jesus shows us in his own life, and in his sacrificial love, what God is like.
I'd just like to add today that, in J.B. Philips' translation, 'The Letter to Jewish Christians' begins as follows:
God, who gave to our forefathers many different glimpses of the truth in the words of the prophets, has now, at the end of the present age, given us the truth in the Son.
In other words, Jesus didn't just tell us the truth, He is the truth.
[Edited May 1st 2007]