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!
All the while dad showed off his famous ripped, hairless legs for the cameras ... of course.
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