If he is an immigrant that was adopted but did not apply for his citizenship, then he is not a citizen yet. You can't circumvent the citizenship requirements with an adoption or a marriage - you can apply for naturalization which allows you to become a citizen after completing all of the requirements.
First, your child must satisfy the criteria set forth in the definition for an adopted “child” in the Immigration and Nationality Act (I.N.A.). These criteria apply to all adopted children, including orphans. Second, your circumstances must satisfy the separate criteria set forth by the I.N.A. for automatic acquisition of U.S. citizenship.
Immigration through adoption, or “Intercountry adoption,” refers to the adoption of a child born in one country by an adoptive parent living in another country. USCIS plays a key role in the intercountry adoption process.
United States immigration law provides three different processes through which someone may immigrate on the basis of an intercountry adoption. An individual may immigrate under one of these provisions only if the individual’s adoption meets all the requirements of that specific process.
Two separate processes apply only to children adopted by U.S. citizens. The child may immigrate immediately after the adoption or may immigrate to the U.S. to be adopted here.
The Hague Process: if the child habitually resides in a country that is a party to the Hague Intercountry Adoption Convention.
The Orphan Process: (non-Hague): if the Hague Intercountry Adoption Convention does not apply.
Many aspects of the Hague and Orphan requirements are similar. To learn details about each adoption process, see the links to the specific process under the “Immigration through Adoption” to the left.
Another process applies to a U.S. citizen or permanent resident who may petition for his or her adoptive child through an Immediate Relative Petition.
Accepted Answer Date Created: October 31,2015