Your son should maintain valid legal status in US. Otherwise he will not be able to adjust his status in the USA. He can maintain his H4 visa status till 21, and then he can change his status to F1. Further, the child status protection act will allow him to file for adjustment of status with you even though he may be over 21 at the respective time when filing.
Emandi Law Firm PC
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New York, NY 10001 Iselin, NJ 08830
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