In responding to your question, I cannot do any better than quoting Muhammad Asad's comment on the above verse. First, let me his rendition of the verse, followed by his note on it, for clarification. The late Muhammad Asad was a convert from Judaism to Islam, who spent decades to the study of the Qur'an.
9:30 (Asad) AND THE JEWS say, "Ezra is God's son," while the Christians say, "The Christ is God's son." Such are the sayings which they utter with their mouths, following in spirit assertions made in earlier times by people who denied the truth!  [They deserve the imprecation:] "May God destroy them!" How perverted are their minds!
Note 44 (Quran Ref: 9:30 )
This statement is connected with the preceding verse, which speaks of the erring followers of earlier revelation. The charge of shirk ("the ascribing of divinity [or "divine qualities"] to aught beside God") is levelled against both the Jews and the Christians in amplification, as it were, of the statement that they "do not follow the religion of truth [which God has enjoined upon them]". As regards the belief attributed to the Jews that Ezra (or, in the Arabicized form of this name, 'Uzayr) was "God's son", it is to be noted that almost all classical commentators of the Qur'an agree in that only the Jews of Arabia, and not all Jews, have been thus accused. (According to a Tradition on the authority of Ibn 'Abbas - quoted by Tabari in his commentary on this verse - some of the Jews of Medina once said to Muhammad, "How could we follow thee when thou hast forsaken our qiblah and dost not consider Ezra a son of God?") On the other hand, Ezra occupies a unique position in the esteem of all Jews, and has always been praised by them in the most extravagant terms. It was he who restored and codified the Torah after it had been lost during the Babylonian Exile, and "edited" it in more or less the form which it has today; and thus "he promoted the establishment of an exclusive, legalistic type of religion that became dominant in later Judaism" (Encyclopaedia Britannica, 1963, vol. IX, p. 15). Ever since then he has been venerated to such a degree that his verdicts on the Law of Moses have come to be regarded by the Talmudists as being practically equivalent to the Law itself: which, in Qur'anic ideology, amounts to the unforgivable sin of shirk, inasmuch as it implies the elevation of a human being to the status of a quasi-divine law-giver and the blasphemous attribution to him - albeit metaphorically - of the quality of "sonship" in relation to God. Cf. in this connection Exodus iv, 22-23 ("Israel is My son") or Jeremiah xxxi, 9 ("I am a father to Israel"): expressions to which, because of their idolatrous implications, the Qur'an takes strong exception.(Quran Ref: 9:30 ) (Muhammad Asad, the Meaning of the Qur'an, pp: 262-63).
Furthermore, the Qur'anic denunciations of the Jews for their acts of disobedience have parallels in the Bible: see, for instance, Psalms 109:17-20, 78:21-22, 69:27-28 and in Mathew 23:33 and 12:34 (cf. Yahya Emerick, The Meaning of the Holy Qur'an in Today's English, p.194).