{Because ye are sons} (hoti este huioi). This is the reason
for sending forth the Son ( 4:4  and here). We were "sons" in
God's elective purpose and love. Hoti is causal ( 1Co 12:15  Ro
9:7 ). {The Spirit of his Son} (	o pneuma tou huioi autou). The
Holy Spirit, called the Spirit of Christ ( Ro 8:9f. ), the Spirit
of Jesus Christ ( Php 1:19 ). The Holy Spirit proceeds from the
Father and from the Son ( Joh 15:26 ). {Crying, Abba, Father}
(krazon Abba ho pat(8872)). The participle agrees with pneuma
neuter (grammatical gender), not neuter in fact. An old, though
rare in present as here, onomatopoetic word to croak as a raven
(Theophrastus, like Poe's _The Raven_), any inarticulate cry like
"the unuttered groanings" of  Ro 8:26  which God understands.
This cry comes from the Spirit of Christ in our hearts. Abba is
the Aramaic word for father with the article and ho pat(8872)
translates it. The articular form occurs in the vocative as in
 Joh 20:28 . It is possible that the repetition here and in  Ro
8:15  may be "a sort of affectionate fondness for the very term
that Jesus himself used" (Burton) in the Garden of Gethsemane
( Mr 14:36 ). The rabbis preserve similar parallels. Most of the
Jews knew both Greek and Aramaic. But there remains the question
why Jesus used both in his prayer. Was it not natural for both
words to come to him in his hour of agony as in his childhood?
The same thing may be true here in Paul's case.