His name is Ariel Antigua, he lives in Lyndhurst, NJ. And according to Edwin Ortiz, the guy who runs Hot Corner training facility, Antigua is "one of the most incredible sights you'll ever see."
Via the Dugout Doctors:
Okay, I'll admit that's impressive, especially seeing a five-year-old hit an 86 mile-and-hour fastball. Of course, all I could think while watching him take batting practice was, THROW HIM A CHANGEUP.