A better translation for the verse,
The Jews say, ‘Uzair is the son of Allah,’ and the Christians say, ‘Christ is the son of Allah.’ That is an opinion that they mouth, imitating the opinions of the faithless of former times. May Allah assail them, where do they stray?!
You asked, 'Is there any historical proof of this outside of the Qur'an?'
You can not find any historical proof that those Jews who were in the time of Prophet Muhammad and believed in that incorret belief complained from Prophet Muhammad that they do not have that belief . So, it could be understood from that verse that there was a group of Jews in the time of Prophet Muhammad that had that belief.
This idea is not odd. You may say that there is not any thing about being son of God( i.e., having a Divine nature ) in Torah, so how did those Jews believe in that thing for Uzair?
But, this can not reject my theory. Because the people of Prophet Moses(pbuh) used to pray idol, even in the time of Moses, their Prophet. Indeed, Torah implies that Jews had an influenced nature and were affected by other polytheists of their times. For example, after many miracles of Moses and the order of God to not making any image and idol for praying it( See, Exodus 20:3, 4 and 5 ), according to Torah, people of Moses asked Aaron to make an idol for them( See
Exodus 32:1 ). Indeed, they were affeted by the theology of polytheists of Egypt who prayed idols, so that acted a doing that the teachings of Moses and the order of God did not let them.
So, it is not odd that some Jews in the time of Prophet Muhammad were affected by the polytheists of their time who before them believed in Divine nature for different beings, so that imitated them and believed in what is against their Holy books. The verse of Holy Quran implies this mimicing from other polytheists( The Jews say, ‘Uzair is the son of Allah' .... That is an opinion that they mouth,
imitating the opinions of the faithless of former times ).
See the correct translation.