How do you say “he is my boss” in Cantonese?

佢= he  系= to be 我 = my/I/me  老=old 细=small
 but 老细=boss.

LearningDict:佢系我老细 ( <<– click and learn the pronunciation and more.

Compare Cantonese & Mandarin ( <<–click and you will see )
他是我老板  ta1shi4 wo3 lao2ban3 is the Mandarin version.