Along comes a one son called a offspring
This is some clever wordplay by Nasir.
Your son is your offspring (also a reference to his past tendency to style himself “God’s Son not to mention the tattoo) and most rappers are in a way Nas' sons as he is one of the most influential rappers alive.
Furthermore, Cole was influenced by Nas and so he was an offshoot, an offspring, of Nas' technique of rapping with his own flavor added.
Also a double entendre that he is referring to himself — When he began, Nas himself was referred to as the second coming of Rakim, so he knows what it feels like to be compared to a prior great and to be “offspring.” This ties to the next line since he starts to tell his journey which suggests that he is the offspring.
To help improve the meaning of these lyrics, visit "Let Nas Down (Remix)" by J. Cole (Ft. Nas) Lyrics and leave a comment on the lyrics box