He became an American citizen the moment he touched American soil...Because of the adoption?
If certain conditions are met the child of a foreign adoption enters the US on a special visa which grants citizenship as soon as they (he in our case) touch American soil. No additional action is required of the adopting parent(s) ... the Certificate of Citizenship arrives automatically a few weeks later.
Because of the adoption?
When he set foot on American soil he obtained dual citizenship. Ordinarily an adoptee has until age 18 to renounce one or the other of his citizenships. I know this because I have an adopted Ukrainian child.