Mets prospect A.J. Ewing showed off his blazing speed when he belted an RBI triple in the seventh inning of his major league debut on Tuesday night in Citi Field. But it was the reaction of his joyous family that stole the show. His dad, Joe, and his other family members were pumping fists, high-fiving and going crazy after Ewing’s big hit in a Mets 10-2 win. In the second inning, Joe talked to SNY’s Steve Gelbs after the rookie worked the first of his three walks, saying he was built for this moment. But Joe admitted he was a “nervous wreck watching him.” Joe, who said he was a high school pitcher who couldn’t hit, did say he was the one who started his son on the path to becoming a left-handed hitter at 3 years old. Joe, who had six messages on his phone before finding out, was surprised his 21-year-old son was called up so soon. “He’s definitely grown as a player,” Joe said.…