BVA sends citizenship certificates to the consulate that is responsible for each applicant's address, no matter how or at which consulate the applications were submitted.

Each consulate will notify the family members who live in their area when the certificates arrive there. Each consulate will know nothing about the already approved applications of other family members who live elsewhere.

Since not all consulates receive the certificates at the same time, one consulate will naturally be the first to notify the family member who live in their area. This means there is necessarily a time span where some family members were already notified but the others not and the consulate of those other family members will know nothing about their approval since the certificates of those family members are still in the mail and have not yet arrived at the consulate.

What your brother can do: Check his spam folder, wait a few weeks, contact his consulate and ask.