How can my son who becomes 21 during the administrative processing of our EB-5 visas enter the U.S.?

He should be protected by the Child Status Protection Act.

Since you were already scheduled for a visa interview when a visa was available to you, you should be safe.

He should be ok, but make sure the consulate knows of the age of concern.

Once you filed the DS-260 and paid the visa fee, the age of your son is frozen.

If you are issued 221-G letter, it means your case is pending and under the administrative procedure, because you are missing some documents or other information. Moreover, it also means that your case has not been finalized and your EB-5 visa petition is still pending even if the child is now 21. Since you filed the petition before the child was filed, his age has been frozen until your petition is approved. Further, if the petition is approved, the child is allowed an additional 2 years to enter the US before he is counted as having being aged out. Thus, as of now, the child has not aged out. Advisably, consult an immigration attorney for further analysis and calendaring.

His age was locked in when you filed the I-526. Unless things really slow down for you, he should be fine throughout the processing.