TB12 and his son, Jack, got a private workout while on tour with Under Armour in Asia this week ... and the 5-time Super Bowl champ hilariously tries to join the fun by grappling with a guy twice his size.

Spoiler alert -- big guy didn't budge. But at least he didn't get sumo powerslammed. We've seen that before ...

Gotta give Brady credit for trying -- he even ditched his shirt to show off his 39-year-old dad bod physique -- but it's pretty weird to see him not dominate something for once.