That is just not true as parents make decisions for children all the time & he had to become an Indonesian citizen just to attend school. Renouncing does not even come into the equation. I never renounced my South African citizenship BUT as I emigrated to Canada with my parents as a young child that decision was made on my behalf & today have no civil rights as a South African as I am in the eyes of the law only entitled as a Canadian.
I don't know what the citizenship laws of South Africa might be.
But I do know the citizenship laws of the United States as they pertain to that period.
A person born on American soil is an American citizen, period. If Indonesia considers him a citizen because he was adopted, that changes nothing -- he is still an American citizen.
And he will stay an American citizen unless and until he renounces that citizenship upon gaining his majority.
Even if he susequently traveled on an Indonesian passport or gained entrance to college as a foreign student, that would not change his status as an American citizen. It might leave him open to some fraud charges, though...