but my confusion is coming from this, i filed the petition when he was 20 years old so he falls under this guideline:
Effect on spouses and minor children: If you filed a petition for your spouse or minor children (under age 21 and unmarried) while you were an LPR, the visa category was family second preference (F2A). When you become a U.S. citizen, NVC will upgrade the petition to an immediate relative (IR) visa category. This benefits your immigrating family member(s) because there are no limits on the number of visas that can be issued each year in the IR categories.
Also, under a federal law called the Child Status Protection Act (CSPA), visa applicants can “opt out” of conversion to the F1 visa category and remain an F2B visa applicant. This may be beneficial because sometimes the wait time for an F2B visa is shorter than the wait time for an F1 visa.
but it appeared that they based they're assessment on this guideline :
Effect on adult children: If you filed a petition for your unmarried adult children (age 21 or older) when you were an LPR, NVC will change the visa category from family second preference (F2B) to family first preference (F1).
We even got an email from NVC saying "Congratulations! The US embassy/ Consulate General agreed to expedite his visa application process, we thought because of the above mentioned reason (effect on spouse minor children) which lead us to biometrics and medical examination only to find out he was denied/ refused??? NVC should've just said "We apologize but your son will be re- categorize and please hold up paying/doing biometrics and medical exam for now, blah blah blah, instead, right?!