Cristiano Ronaldo -- Adorable Ab Workout ... With 4-Year-Old Son

Cristiano Ronaldo's 4-year-old son is already taking after his father ... working on his abs during a father/son training session ... and it's friggin' adorable.

The kid -- Cristiano Ronaldo Jr. -- started off by holding his dad's feet while the soccer star ripped off a set of crunches.

Then the tables turned ... with dad pushing his kid to knock out a solid 10 sit-ups ... which he does without a problem!