Before the Child Citizenship Act of 2000 took effect on Feb. 27, 2001, adopted children did not qualify for N-600; their adoptive parent(s) had to apply N-643 for them, and they did not obtain citizenship until the N-643 was approved, the approval of which required an oath. And it had to be approved before their 18th birthday otherwise they wouldn't get citizenship (but as an adult they could then apply for citizenship on their own via N-400).
You apparently did not complete the N-643 process, so your son would not have obtained citizenship through that.
However, your son may have obtained automatic citizenship on or after Feb. 27, 2001 under the Child Citizenship Act. Was he still under 18 on Feb. 27, 2001? If yes, was he living in the US in your physical and legal custody (I'm assuming you're a US citizen) at any time between Feb. 27, 2001 and his 18th birthday? What was done with his green card, do you at least have a copy of it? Is he 18 or older now? Does he have a US passport?
Your answers will determine the options for what to do next.