If he can prove that the first soldier is his blood-father, and that that father spent the required amount of time in the US prior to his birth ... then he is a US citizen and can file an N-600 to get proof.
This may require getting documents from the birth father's military and education files. Are they in contact?
Hiring a lawyer and, possibly a private detective, may be a wise thing to do.
Otherwise, naturalization might be the easiest way to go.
PS Spending lots of time in the US and paying taxes is irrelevant.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
Your husband is a US citizen. Hire an immigration lawyer help you fill out an N-600 and come up with the necessary proof to be proved with it.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
You must check the Nationality Chart for Citizenship. If he qualifies as an automatic citizen then he cannot file for N-400 since he is already a citizen. You cannot naturalize someone if they are already a citizen (i.e. automatic citizen). Hire an experienced immigration lawyer to help you.
Garmo Law Group, PLLC (Michigan) 248-626-0050. This advice is only general in nature and does not constitute an attorney/client relationship. Speak to an experienced attorney before making decisions.
I concur with my colleagues.
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If his father was not married to his mother, he is not a derivative US citizen from the father. If his mother did not naturalize before he turned 18, he is not a US citizen from her either, so he would have to file to naturalize.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
As you can see, different attorneys have guessed differently. What you must realize is that citizenship is actually a complex topic. It does require research. Immigration and Naturalization and Citizenship laws have changed a number of times since your husband entered the US and are dependent on various dates and calculations. I would agree that you should hire an attorney to help answer this question and file the appropriate paperwork.
Dhenu Savla, Esq.
This answer does not create an attorney-client relationship and is not meant to be relied upon as legal advice.