A.T. Robertson Commentary John 1:49

"Nathanael answered him, Rabbi, thou art the Son of God; thou art King of Israel." — John 1:49 (ASV)

Thou art the Son of God (συ ε ο υιος του θεου). Whether Nathanael had heard the Baptist say this of Jesus (1:34) we do not know, apparently not, but Nathanael was a student of the Old Testament as Philip implied (1:45) and was quick to put together his knowledge, the statement of Philip, and the manifest supernatural knowledge of Jesus as just shown. There is no reason for toning down the noble confession of Nathanael in the light of Christ's claim in verse 51. Cf. the confession of Peter in 6:69; Matthew 16:16 and Martha's in Joh 11:27. Nathanael goes further.

Thou art King of Israel (Βασιλευς ε του Ισραηλ). To us this seems an anti-climax, but not so to Nathanael for both are Messianic titles in Ps 2 and Jesus is greeted in the Triumphal Entry as the King of Israel (John 12:13).