The Jews say "Uzair is the son of God" בן אלוהים

[9.30]

وَقَالَتِ الْيَهُودُ عُزَيْرٌ ابْنُ اللَّهِ وَقَالَتِ النَّصَارَى الْمَسِيحُ ابْنُ اللَّهِ
The Jews say: "Ezra ('Uzayr) is Allah's son," and the Christians say: "The Messiah is the Allah's son."

According to Jewish tradition, being "son of God" means that the son receives from the father, the Most High, and it is an election, that made to the sons of Israel, hence the prayer "Hear Israel", God speaking to his son, the 12 tribes.
Most of them refused to listen to the Most-High, going so far as to secede from the proto-Kingdom of Israel established with David, now divided between the Kingdom of Judea and that of Samaria. After this event, only the subjects of the Kingdom of Judah will be considered "sons of god", whom we call Yahudis.

Who is U'zair, and considered by the Yahudis as one of their own, a Son of god ? When applied to a particular individual, then it means Messiah :

Psalm 29.1

מִזְמוֹר לְדָוִד: הָבוּ לַיהוָה, בְּנֵי אֵלִים; הָבוּ לַיהוָה, כָּבוֹד וָעֹז
A psalm of David, son of Elohim, you heavenly beings, ascribe to God glory and strength.

The Messiah also has another particularity, he builds the Temple for worship and sacrifices, the Temple of Jerusalem. This is what King David undertook, and his son Solomon, who had less blood on his hands, finished building it. The Temple and its construction, a fixed idea in rabbinic Judaism.
Their prophet Jesus also bears the name Messiah son of God, but he was betrayed by his own people who rejected him. They want a builder as Messiah, but the one who came to them didn't need to build the Temple, rather he announced its destruction, for having collaborated with the tyranic Roman occupants, and in addition wanting to denounce him to the pagans occupiers of holy land :

Matthew 29.1-2

Jesus left the temple and was walking away when his disciples came up to him to call his attention to its buildings.
“Do you see all these things?” he asked. “Truly I tell you, not one stone here will be left on another; every one will be thrown down.”

So we have seen that Son of God addressed to a particular person automatically means Messiah and builder of the Temple, but yet another characteristic, he is the shepherd and bearer of the Promise of salvation and he recieved strength :

Psalm 28.8

יְהוָה עֹז-לָמוֹ וּמָעוֹז יְשׁוּעוֹת מְשִׁיחוֹ הוּא.
God is a strength unto them; and He is a stronghold of salvation to His Messiah.

Who was this U'zair, son of god, Messiah, sheperd and bringing the promise of salvation ?

Isaiah 45.1

כֹּה-אָמַר יְהוָה, לִמְשִׁיחוֹ לְכוֹרֶשׁ אֲשֶׁר-הֶחֱזַקְתִּי בִימִינוֹ לְרַד-לְפָנָיו גּוֹיִם, וּמָתְנֵי מְלָכִים, אֲפַתֵּחַ--לִפְתֹּחַ לְפָנָיו דְּלָתַיִם, וּשְׁעָרִים לֹא יִסָּגֵרוּ.
Thus saith the God to His Messiah, to Cyrus, whose right hand I have holden, to subdue nations before him, and to loose the loins of kings; to open the doors before him, and that the gates may not be shut:

Isaiah  44.28

הָאֹמֵר לְכוֹרֶשׁ רֹעִי, וְכָל-חֶפְצִי יַשְׁלִם; וְלֵאמֹר לִירוּשָׁלִַם תִּבָּנֶה, וְהֵיכָל תִּוָּסֵד.
That saith of Cyrus: 'He is My shepherd, and shall perform all My pleasure'; even saying of Jerusalem: 'She shall be built'; and to the temple: 'My foundation shall be laid.'

Yes, the Messiah of the Yahudis did not come from the family of Jacob, father of the twelve tribes of Israel, but from a Persian king, a king who dominated the Middle East, and where the Yahudis resided since their deportation to Babylon.
Not being Yehudis, what were the discussions among the Yahudis? Would he have been accepted by everyone as the son of god without being part of any tribe? This debate was most likely intense, and it took the efforts of scribes like Ezra to establish him as the Son of God; the beloved of God.

Morphosemantic:

The word عُزَير from root عزر comes with the pattern which means affection, a love, and the etyma عز means the Power, in arabic aswell in hebrew.
In hebrew, the root عزر means the assistance through tool, the utility, something, or someone, bringing support, aid, for a task, something to upgrade the power.

Isaiah 45.1

כֹּה-אָמַר יְהוָה, לִמְשִׁיחוֹ לְכוֹרֶשׁ אֲשֶׁר-הֶחֱזַקְתִּי בִימִינוֹ לְרַד-לְפָנָיו גּוֹיִם, וּמָתְנֵי מְלָכִים, אֲפַתֵּחַ--לִפְתֹּחַ לְפָנָיו דְּלָתַיִם, וּשְׁעָרִים לֹא יִסָּגֵרוּ.
Thus saith the God to His Messiah, to Cyrus, whose right hand I have holden, to subdue nations before him, and to loose the loins of kings; to open the doors before him, and that the gates may not be shut:

..."whose right hand I have holden", Allah is using the hand of Cyrus as a tool of Power (over nations), meaning of عزر .

Isaiah  44.28

הָאֹמֵר לְכוֹרֶשׁ רֹעִי וְכָל חֶפְצִי יַשְׁלִם וְלֵאמֹר לִירוּשָׁלִַם תִּבָּנֶה וְהֵיכָל תִּוָּסֵד
That saith of Cyrus: 'He is My shepherd, and shall perform all My pleasure'; even saying of Jerusalem: 'She shall be built'; and to the temple: 'My foundation shall be laid.'

..."(Cyrus) shall perform all My pleasure... It indicates here again the meaning of tool, root meaning of عزر , and a pleasure, the pattern meaning used on the root.

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To say that the redacter of the tanakh could be the one quoted in the Qur'an was a wrong track, even if עֶזְרָא (Ezra) is formed by the same root, with an A suffixed. Nowhere it's written that Ezra is Messiah, nor any of the caracteristic, inclued son of god.
But also, I found no proof that Cyrus was called Son of God by the jews.

Another interpretation of the Qur'anic verse, this U'zayr could be the one the jews are still waiting for. In this surah context, the verse opposed strickly the christians and jews claim, their pretention one over the other.

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