DIDDY PASSIN' THE COURVOISIER With Bieber At Son's 21st

DIDDY

PASSIN' THE COURVOISIER

With Bieber At Son's 21st

EXCLUSIVE

Couldn't nobody hold Diddy or his son down this weekend -- the two were drainin' bottles (of Ciroc of course) at Justin Combs' 21st birthday.

Diddy's definitely a cool dad ... the rapper threw his son an epic '90s-themed party at a mansion in Bel Air for all his friends along with celeb drop-ins like Justin Bieber, Khalil, Kendall Jenner, Madison Beer, and A$AP Rocky.

Check out the photos from inside the bash ... not sure if the Statue of Liberty was a b-day gift or a party guest. Moment of the night -- we're told a boozy Diddy toasted his son and bragged about JC's enormous trust fund. 

Mo Money ... NO problems