This is where understanding of the biblical Greek phrase, about Jesus being "the only-begotten" Son of God is important. Sadly, some modern translations of the Christian Greek scriptures have watered this down, by replacing that phrase with "God the One and Only" (NIV) or "the only-begotten god" (NWT with a small 'g') or "the only Son" (GNB) etc.
Older translations, like the Authorised Version, stick to the Greek text which has 'monogenes', applying to the Son (Jesus Christ), and so reading "the only begotten Son, which is in the bosom of the Father, he hath declared [God]." Notice the point about the position of the Son in relation to the Father? The whole verse speaks of the unique relationship of this Son to the Father. No other son of God is in the bosom [position - relationship] with God the Father.
That is because this only-begotten Son was never created. He is the eternal Son by virtue of "all the fulness of God dwelling in him bodily" as Colossians 1:15-19 shows. This Son was previously known as the Word of God, who was with God in the beginning, who IS God, and who made everything that was made - proving that he could not have been made (created) himself. That is the uniqueness that John 1:18 enlarges on. If you don't take the first 17 verses of John chapter 1 into the reading, the meaning of verse 18 will remain obscure. And especially so if you use modern translations, which detract from the unique relationship of this Son with God the Father.
The relationship that the many other sons of God enjoy is different to that of the only-begotten Son. Angels are called sons of God, but although they enjoy being in the very presence of God in heaven, they are created sons. Likewise with human sons of God. All humans are created by God and could be said to be 'sons' in that sense, but the Christian Greek scriptures speak of how repentant ones who have faith in Jesus become adopted into God's heavenly family. "Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is." 1 John 3:2. John was addressing that to fellow-Christians who were expecting the return of the Son of God in great glory, to transform them with resurrection bodies, to be with him forever.
This means that the answer to your question is that Jesus is the uniquely, singularly uncreated Son of God, the eternal Son, which can never be said of all the other sons of God. John 1:18 (when translated accurately) brings out the unique relationship of Christ (as the only-begotten Son).
Romans 14:8 does not detract from this because that verse applies only to humans who have first become adopted children of God. The very next verse explains that those ones
"have received the Spirit of adoption, whereby we cry, 'Abba, Father!'
The Spirit itself beareth witness with our spirit that we are the
children of God." (Romans 8:15-16)
But prior to that miraculous adoption, they were NOT children of God. There came a point in time when God chose to adopt them as his children, unlike the eternal Son of God who was always in the bosom of the Father, from before the beginning.