In researching the subject of 1 John 5:20 and the Greek pronoun οὗτος, I have checked grammatical works by Smyth, Blass-Debrunner and Funk, J. W. Wenham, and Moulton-Turner, among others. Only Moulton-Turner proves to be helpful here (IMO)--I might add Winer to the list as well.
On page 44 of Moulton-Turner (Vol. III, Syntax), we are told that οὗτος appears frequently in the papyri and NT. It can refer to someone present (Luke 15:30), and may also refer to "the noun which is most vivid in the writer's mind."
Examples given after this statement are Matt. 3:3; 3:17; Acts 4:11; 1 Jn. 5:20. Concerning 1 Jn. 5:20, we read: "God, not Christ, is the true God" (Moulton-Turner, p. 44). Looking up 1 Jn. 5:20 in the older Interpreter's Bible, I found out that it too identifies the Father as the subject described by οὗτος. He is the main subject of 1 John 5, and the Father is the true God and life everlasting (cf. Jn. 17:3; 1 Jn. 5:11).
Just because Moulton-Turner, the IB, Winer, Zerwick, Westcott, Murray J. Harris and others say that the Father is represented by οὗτος doesn't make the case absolutely ironclad. But I thought you might appreciate the information found in Moulton-Turner.