anarthrous[1] “son” in Hebrews 1:2

Heb. 1:1 Πολυμερῶς καὶ πολυτρόπως πάλαι ὁ θεὸς λαλήσας τοῖς πατράσιν ἐν τοῖς προφήταις  2 ἐπ᾿ ἐσχάτου τῶν ἡμερῶν τούτων ἐλάλησεν ἡμῖν ἐν υἱῷ, ὃν ἔθηκεν κληρονόμον πάντων, δι᾿ οὗ καὶ ἐποίησεν τοὺς αἰῶνας·

You will find several explanations in the literature for the lack of an article with υἱῷ, with one that struct me initially as quite plausible: This is the first mention of the “son” in Hebrews, therefore  it is new information. This explanation presupposes a code model of communication. It looks at the cotext[2] and determines that υἱὸς is unknown to the intended audience at at Heb. 1:2 since it is the first appearance of the lexeme.  

Adopting an ostensive-inferential language model changes the picture. R.A. Hoyle argues that υἱῷ is discourse new but hearer old because the author assumes his audience will be able to uniquely identify the referent of υἱῷ the first time it is mentioned. Think of a pastor launching his sermon with the statement  ὁ θεὸς … ἐλάλησεν ἡμῖν ἐν υἱῷ. Hoyle[3] explains:

Hebrews begins without a typical letter opening. The letter’s theme is marked by the anarthrous use of “son” in Hebrews 1:2:

ἐπ᾽ ἐσχάτου τῶν ἡμερῶν τούτων ἐλάλησεν ἡμῖν ἐν υἱῷ

in last of-the days these He-spoke to-us in Son

Given the context of a letter to Jewish Christians, “son” here is clearly intended to have the “Hearer-old” reference to God’s Son, Jesus Christ, so in unmarked use the article would be expected. Lack of the article then is statistically unusual and hence salient.

The word “son” at first sight appears to be contrasted with “the prophets” in 1:1, but whereas the prophets are not a key theme in Hebrews (hence have the article marking Hearer-old and appear in a Participial clause), the son is marked salient as a key theme of the whole letter. The thematic salience of “the Son” at the highest Discourse level, the whole letter, is clearly justified from the wider context, e.g. “son” occurs 21 times in Hebrews, of which 12 (13 including 2:6 as exegeted by the writer of Hebrews) refer to Jesus. (Only the Gospels and 1 John have more occurrences). Also the Son is contrasted explicitly with angels (1:2–2:18), with Moses (3:1–4:13, especially 3:6), and with high priests (3:14–10:39, especially 7:28 and 10:29). Moreover, Jesus, to whom the son in 1:1 refers is also clearly thematic at TEXT level, e.g. 2:9, 3:1, 4:14, 6:20, 10:19, 13:20–21, etc. — Richard A. Hoyle
    

[1] anarthrous, without an article

[2]Cotext refers to the surrounding text in the document under consideration. Context refers to the total cognitive universe of the author and his assumptions about the cognitive universe of his audience.

[3]  Richard A. Hoyle, Scenarios, discourse and translation, SIL 2008, p.735.