If his Indonesian step father adopts B, he reverts to the Indonesian citizenship. Check the adoption laws of the US. You are right, parents cannot take away your US status, however adoption changes the situation. Otherwise, if BO is 18 years old, can the US draft him for military service (assuming US draft still existed) and arrest him for draft dodging and come after him for any US obligations if B never left Indnosia????
Nothing a man in Indonesia to/for a US citizen can remove his citizenship. Such a proceeding according to Indonesian law has no impact on his natural born status if it exists.
US law trumps Indonesian law including its adoption law.
Selective service law might have some claims on an adopted citizen but they would be tenuous and doubtful if he were never a resident of the U.S.