Tom Cruise's Son -- Beached ... in Mexico

Connor Cruise hit the beach on the Mexican Riviera ... and was almost unrecognizable with dreadlocks and a chinstrap.

Tom's son is 19 now ... and looking much different from the kid we used to see around NYC and L.A.

Connor was down in Mexico for the grand opening of a Hard Rock Hotel and Casino -- and of course Cinco de Mayo.

CC's been blowing up as a DJ -- we shot him about a year ago after a gig in L.A. ... and the difference is amazing.