Immigration Status of Adopted Child

Hi everyone; My son is a US citizen living in France married to a French wife. They adopted a Mexican child under the Hague convention procedures. The adoption was finalized six months ago and the child now has a French passport. My son has been recalled to the United States and will returning to live here with his family. His wife has a green card but in researching the status of the child we find that the child cannot get a visa because he has been adopted for less than two years. Can this be true? Can the immigration laws force a separation of the family until the two years are up? Needless to say we are very worried about this.Any advice would be appreciated. Thanks