Alex Calvillo and Savannah Ybarra of Indiana say they first became worried about their youngster's whereabouts after the boy, named Aziel, disappeared as they prepared brunch for loved ones.

Aziel, who has been diagnosed with autism, was left on the couch to watch a tablet ... but wandered off -- which his parents told WPTA-TV he tends to do.

As the parents began searching for their little one Sunday, concern grew due to the fact that even neighbors had yet to spot him in the neighborhood.

That's when Savannah decided to call the police for backup ... and is relieved she did, as Fort Wayne Police Officer Evan Myers eventually discovered Aziel in the pool of a neighboring home.

Myers' body cam footage from the scene shows the cop's quick response to hearing Aziel crying out for help. Watch the video ... he's seen running across a yard and through an open fence to find Aziel floating face-up in an above-ground pool.

Myers quickly fished the young boy out of the water, reporting to his squad via his walkie that he had found Aziel, who appeared upset but unharmed.

Aziel's dad wasn't surprised to learn his son ended up in the swimming pool ... noting his kid is a big fan of water -- a fact police didn't know until just before the rescue.

Alex added ... "After I reminded him that he’s autistic and likes water, about like two minutes after that, they ended up finding him."

Both Alex and Savannah offered up their thanks to Myers ... noting it meant a lot to their family to see their son safely brought home.