According to the latest visa bulletin (May 2024), the Philippines is the only country where F1 is more advanced than F2B. Indonesia falls into “All chargeability areas.” You should notify NVC because if you do not, the consul will discover it during the interview and then the case of your son will be put in administrative process while they ask for DHS authorization on your behalf. It would be better to do that in advance (send an inquiry to NVC). However, wait until the priority date, Sep 2016, is current in the chart A, whether it is in F1 or F2B, to take action (Opt out of the conversion to F1 and remain an F2B visa applicant).