Liam Attridge, Ricky Garcia and Emery Kelly, the wildly talented teens who make up the “forever” part of Forever in Your Mind, will never forget it. During FIYM’s recent Boston concert, they asked fans to take out their phones and turn on the flashlights. In an instant, 2,000 points of light illuminated the hall like a starry sky. “It was so beautiful to see them respond like that,” recalls Liam. They’d better get used to it.