If Form I-526 or I-526E is filed before his 21st birthday, the Child Status Protection Act freezes his age for the time the petition is pending. Whether he remains a child when a visa becomes available depends on the arithmetic at that point, so file early.
The filing date starts the age protection
Derivative children must be unmarried and under 21 when they immigrate, but the Child Status Protection Act subtracts the time the investor petition was pending from the child's age on the date a visa number becomes available. In a case where the petition takes two years to adjudicate and the son is 22 when it is approved, his adjusted age is 20 and he remains eligible, provided he seeks to acquire the visa within a year of availability and stays unmarried. If the petition is filed after he turns 21, there is no protection at all and he would need his own immigration route.
The practical consequence is that the family should not wait for a better project or a better exchange rate if the delay pushes the filing past his birthday. Confirm the current pending-time practice for I-526E cases and check visa availability in the Department of State's monthly bulletin, since retrogression in the set-aside categories can lengthen the wait and alter the calculation.