Thank you for your question. You are the final stages of the process. All that is left at this point based upon your question is the completion of the DS-230 along with the I-864 Affidavit of Support if the application has not already been completed.
Your step-son would complete the DS-230 immigrant visa
application, have an interview, and if approved, he would enter the US as a lawful permanent resident of the United States.
Although your step-son has turned twenty-one, it is very likely that he would still be eligible under the Child Status Protection Act. As for an exact time frame, it is difficult to estimate within this forum but it is very likely that an immigrant visa appointment will be scheduled within the next thirty days.
The Child Status Protection Formula is below for your reference:
1.Child's Date of birth:
2.Receipt Date of Petition Filed:
3.Date the Form I-130 was approved:
4.Length of time the Form I-130 was pending:
5.Date the petition became current (by looking at the Department of State Visa Bulletin):
6.Date the visa became available (later of #3 or #5):
7.Age of daughter on date visa became available: (#6 minus #1)=
8.Age of daughter for CSPA purposes ( age at time visa became available minus length of time petition was pending (#7 minus #4):
In any event, it is probably time to retain an immigration
professional to ensure a smooth transition for your step-son.