DoS takes the position that U-3 derivative applicants must be under 21 at the time the visa is issued. It sounds like DoS never issued your son a visa, and if that is the case he will be unable to obtain the U-3 visa. It would be worthwhile for you to consult with an immigration attorney and bring as many of you and your son's immigration documents as possible to evaluate what options he may have.
Come armed with all the facts such as date of entry, manner of entry, any immigrant visas filed in the past, as well as approval notices and receipts.
This is a general answer only and does not imply that I am your attorney giving advice with full knowledge of all the particulars of your case or that there is any attorney client relationship. I strongly urge you to retain experienced legal counsel who can better advise you once they have reviewed all your documentation and are fully apprised of the details of your case. Rebecca Black Immigration 5800 Beach Blvd. Ste 203-176 Jacksonville, FL 32207 904-999-4928 Tel & Fax
I agree with Mr. Clopman.
While this answer is provided by a Florida Bar Certified Expert in Immigration and Nationality Law, it is for general information purposes only and an attorney/client relationship is neither intended nor created. You should seek out qualified counsel to review your case and provide you with advice specific to your situation. Call +1-561-478-5353 to schedule a consultation.