He's a US Citizen. Legally they cannot deny him entry into the country.

Central to these protections is the absolute right of a U.S. citizen to return to their country. This right is unequivocal. No matter where an individual has traveled, the duration of their absence, or their personal or legal background, a U.S. citizen cannot be denied entry into the United States. This principle is supported by both statutory and constitutional law. A landmark Supreme Court case, United States v. Wong Kim Ark (1898), established that citizenship includes the right to reenter the nation. Unlike lawful permanent residents or visa holders, U.S. citizens are exempt from admissibility criteria. Consequently, even if a U.S. citizen lacks proper documentation or is subject to further questioning, the government is obligated to permit their entry once their citizenship is verified.