From Stephen Smalley's Word Commentary on 1 John:
"the most natural way of construing hOUTOS in v 20 (which need not refer to the nearest antecedent, and may allude to the main subject of the preceding statement as a whole; cf. 2:22; 2 John 7) is to take it as a reference to God: the God whom we recognize as genuine through the insight given us by his Son, and with whom we are in fellowship through Jesus Christ. 'This is the real God.' It is precisely through knowing him, as the Gospel [John 17:3] maintains, that eternal life itself becomes a reality" (p. 308).
Smalley does go on to write that if hOUTOS in 1 John 5:20 in fact refers to Jesus, then "we are presented with a NT christological witness which is rare in the NT." He cites John 20:28 as clear proof that Jesus is called God. Romans 9:5 is disputed, he says, and "Titus 2:13 is uncertain, since the Gr. can either mean, 'our great God and Savior Jesus Christ,' or 'the great God and our Savior Jesus Christ' " (Smalley 308).
Lastly, Smalley concludes that the writer of 1 John 5:20 may be ambivalent in this passage, but all of these remarks must be considered in the light of the initial statements I cited: the most natural way to construe hOUTOS is with TON ALHQINON.
A friend who uses the name "Martin Smart" provides these remarks on 1 John 5:20:
Zerwick, page 733 In A Grammatical Analysis of the Greek New Testament by Max Zerwick, Mary Grosvenor (4th edition) they say regarding 1John 5:20 with regards to hOUTOS
"the ref. is almost certainly to God the real, the true, op. paganism (v21)."
I have confirmed this quote in my personal copy of Zerwick and Grosvenor.