Tim Walz's Proud Son Cries During Dad's Speech at DNC

Gov. Tim Walz's big moment at the DNC brought the crowd to its feet, especially his 17-year-old son ... whose emotional reaction to his dad's speech became a real tearjerker moment.

Gus Walz sat front and center Wednesday night in Chicago, and he just couldn't hold back his emotions in the moment -- breaking into tears and screaming "That's my dad!" during the speech.

Check out the clip ... Gus -- who has a non-verbal learning disorder, ADHD and anxiety -- jumped to his feet clapping loudly and pointing with his mother Gwen and sister Hope sitting beside him.

Gus got even more emotional after climbing onstage ... wrapping his dad in a bear hug before he and his family raised their arms in a triumphant pose.

Now, Gus is getting as much attention as his VP-hopeful father ... at least online, where many are applauding his heartwarming enthusiasm -- and, naturally, Harris-Walz supporters say it's a sign of how beloved the Minnesota Governor already is, and will be as VP.

Worth noting ... Walz talked openly during his speech about the fertility treatments he and his wife went through to conceive Gus and Hope -- a point of contention among some Republicans who are carrying around fake J.D. Vance sperm sample cups.