Rob Kardashian Watchin' Blac Chyna Put in Werk, Werk, Werk

Rob Kardashian's birthday came early ... as Blac Chyna put on a booty popping clinic for him ... and several hundred other onlookers.

Rob was grinning from ear to ear Sunday night in a Montego Bay nightclub where BC was hosting. Sporting a black tee and shorts, he's still not revealing much of his slimmer physique -- but the smile kinda says it all.

Rob turns 29 on Thursday. Enjoy the ride!