As @Greek2Me said above, it would be illogical to interpret "Son of God" of John 1:49 as @seedy3 does, as simply believing that Jesus is the Messiah and king of ordinary Jewish expectations - just a political leader to liberate and exalt Jewish nation. Why?
Because, what Nathanael witnesses is something extraordinary: a man, who is from Nazareth, and who thus deserves his utmost skeptical and even suspicious attitude as an impostor falsely claiming Messiaship about himself, tells him something which only he, Nathanael could know, namely, his inner core, his inner uprightness, integrity and lack of any double-dealing ("a true Israelite in whom is no deception"). In fact, Nathanael could have thought that this possible impostor simply praises him for winning his good disposition and not really knowing him, and that's why he checked Jesus: "how do you know me?" The response of Jesus blew his mind, for this response (whatever good and beautiful apocrypha exists there, does not matter, for even John's text itself is sufficient for delivering the point) entailed the proof that Jesus really knew his heart's core, his inner thoughts and dispositions, for in "I saw you", is not implied a physical sight, for you can see a person physically and will still be totally unable to know what is in his heart, but in the "saw" is implied that He saw Nathanael's heart, his thoughts while he was standing under the fig. Of course, fig is clad with semantics of Adam and Eve's expulsion from paradise, for they covered their nakedness with fig leaves, and it is justifiable to opine that perhaps Nathanael was thinking like "will the Messiah only reinstate the Kingdom of Israel, or will a true Messiah also introduce an ontological change for all mankind as to lead us back to paradise?" But even if this is a plausible and theologically justified over-interpretation of or eisegesis in the text, we cannot be sure of its objectivity on the given textual basis itself. But what is sure, Nathanael got convinced that Jesus has an authority on His own ("I saw you" and not "God revealed to me" or "angel whispered to me" etc.) to know human hearts, and this is a prerogative not of any even of the highest-ranking angels, but only of God (cf. Psalm 94:11; 1 Kings 8:39 etc.) and the one who is equal to God in nature, and exactly this equality and perfect unity of nature is implied in the appellation "Son of God", for as "son of human" is 100% human in nature, so also "Son of God" is 100% God in nature, with a difference that human begets a son in a process, but in God's nature there is no process, and thus God's Son is eternally so.
Therefore, the phrase "ῥαββί, σὺ εἶ ὁ υἱὸς τοῦ θεοῦ" is a declaration of divinity of Jesus.