Walter Kasper on the Son of God Concept in the Old Testament

The following is taken from Jesus the Christ (page 164):

"Although the Old Testament uses the title of Son for
the people of Israel (cf., among other texts, Exod
4.22-3; Hos 11.1), for the king as representative of
the people (cf., among other texts, Ps 2.7; 2 Sam
7.14) or - as in late Judaism - for any devout and
righteous Israelite (cf., among other texts, Ecclus
4.10), this usage is not based either on the
background of mythological-polytheistic thinking or on
the pantheistic background of Stoic philosophy,
according to which all men in virtue of their common
nature have the one God as Father and are therefore
called sons of God. The title Son or Son of God in the
Old Testament must be understood against the
background of election-faith and the theocratic ideas
based on it. Consequently, divine sonship is not
founded on physical descent, but is the result of
God's free, gracious choice. The person so chosen as
Son of God receives a special mission within salvation
history, binding him to obedience and service. The
title of Son of God therefore is understood, not as
natural-substantial, but functionally and personally."

I would add that the title "son of God" is also used as a scriptural metaphor for Jesus Christ and others.