But he was also born a U.S. citizen and India doen’t allow dual citizenship, so which citizenship holds?
He is entitled to be considered a citizen of India and India would have jurisdiction. Birth right citizenship is not an American law. It is a regulation of the US Citizenship & Immigration Services.
That is why President Trump & several other candidates have included the removal of this policy as part of their platforms. I do not believe Indian citizenship could be renounced until adulthood since there are legal forms in the process.
https://www.uscis.gov/policy-manual/volume-12-part-h-chapter-3