J.J. Watt -- Returning the Favor to Awesome Kid Fan

J.J. Watt received an awesome gift from the coolest 7-year-old on the planet ... and TMZ Sports has learned little Anthony Tarantelli can expect something in the mail from #99 real soon.

Tarantelli captured the hearts of every sports fan when he sent Watt an autographed jersey (not the other way around) and included a letter saying he was "the most feared rusher in my league this year."

Anthony's dad Joe tells TMZ it took a little luck to get the delivery to Watt. Joe says he posted about it on Facebook ... and a friend came forward saying he knew Watt's agent.

Lauren Tarantelli, Anthony's mom, tells TMZ Sports ... J.J. reached out to Anthony after getting the package and sent along a video message ... in which he said Anthony can "expect something in the mail."

Mama Tarantelli -- who was a college tennis player herself -- says Anthony's love of J.J. came about all on his own (his dad's a Bills fan) and that he's so feared in his league ... he once had the referee tell him to go easy.

Perhaps NFL refs should try that approach with J.J.