Forget Rookie of the Year ... Darius Leonard is now the Son of the Year -- 'cause the Colts superstar just bought his mom a new car ... AND SHE FREAKIN' LOVED IT!!!
Leonard surprised his mom with a new Mercedes over the weekend ... and her reaction to the gift was priceless -- doing a FULL SPRINT to the car, followed by celebratory laps in the parking lot.
It's an amazing video.
"Ain't [nothing] like putting a smile on your mother face!" Leonard wrote of the present ... "We've been through so much and now it's like we're on top of the world!