When a custodial parent is no longer seen as fit by the authorities the other biological parent is called into court (May be the reason for Srs visit at age ten...family matters IIRC was given as the reason)
If custody/guardianship is awarded to another family member the agency would continue to keep tabs on the child until reaching the age of majority for the childs own protection.
You are attempting to apply American standards in handling child abuse problems to Indonesia, a predominantly Muslim country. That's like trying to put a round peg in a square hole. Indonesia wouldn't intervene in such situations, especially when the stepfather is a Muslim. Thus it's impossible for me to find any plausibility to your speculation.
But Catholic sisters would intervene. Particularly if the boy asked for help, ran away or otherwise let them know he wanted or needed to go to his grandmother. Remember His mother spent little or no time with him from the time he came home to HI and became an adult. There had to be a reason for that.