Sorry, Dana, I somehow didn't see this post of yours until today.
In the scene you're referring to, Matsuoka is talking to the kids (including Kei aged 11) who have just lost a match during "Shuzo Challenge," an annual summer tennis camp for young boys. He's yelling that he can't understand why the kids don't show their frustration more after losing a match and that he's feeling more gutted. The camera then focuses on Kei probably because he was already the most promising of the bunch. So it's not that Matsuoka is yelling only at Kei.
Matsuoka is a passionate guy and is not shy about showing his emotion on court. Many Japanese kids tend to be shy and Matsuoka thinks they should be more extrovert on court.
In the scene before that, however, he tells the kids they shouldn't look dejected after losing a point. He says they should walk like after any other point. And he talks to Kei, "It's difficult for you now, Kei, huh?" And Kei answers, "I can do that." A sweet little thing he was!
In another scene, you see a 12-year old Kei play a practice match with a taller boy, a high school student, and Kei won.
And as you probably guessed, the middle aged couple seen just after the scene of Kei's victory at Delray Beach are his parents.
His dad says, "It's a miracle," and his mum says, "He likes karaage (Japanese-style fried chicken) so I want to cook for him a plateful of karaage."