Can he apply for a visa in Brunei? Kindly let me know. Thank you :)

Bryan, many Chinese embassies usually follow the same rule. If citizens from the third countries have resident permits (more than six months), they usually can apply in the third country. However, several embassies also handle the applications for those who just have short term entry permits.

You'd better ask your son to visit Chinese embassy and confirm what policies Chinese embassy in Brunei adopts.