Here's Tremper Longman III on why "son" is not capitalized in the new NIV translation of Psalm 2.
Question: What difference does it make if we capitalize ‘son’ in Psalm 2?
Well, the difference that it makes is that if you capitalize son in Psalm 2 it shows you don’t understand the psalm. The son in Psalm 2 is the Davidic descendant who assumes the throne. [The psalm] was likely sung at inauguration services and other royal ceremonies. We can see this by the allusions to 2 Samuel 7, which speaks of David having a son on the throne forever.
Of course, as readers of the New Testament we know that Psalm 2 has a deeper significance that probably wasn’t known by its original composer or audience. After the monarchy failed, the faithful realized that the fulfillment of 2 Samuel 7 was not found in the line of Davidic descendants whose rule came to an end in 586 BC. Thus, particularly in the late Old Testament time period and into the Intertestamental period, the eschatological significance of the Davidic covenant and the royal psalms were emphasized. Jesus is the greater son of David who is the ultimate fulfillment of the Davidic covenant and also of course of Psalm 2 as the numerous references to the psalm in the New Testament indicates. However, Psalm 2 is not a messianic prophecy, which would be the only reason to capitalize son in this psalm.
I am utterly speechless!