How sunny and well-meaning and therapeutically feel-good is the new autism drama “The Unbreakable Boy?” It’s a movie in which the dad, played by Zachary Levi, has an imaginary best friend. It’s a movie in which the local church is run by a Matthew McConaughey sort of dude named Preacher Rick (Peter Facinelli), who’s so down with his downtrodden flock that he himself is a recovering alcoholic who admits he sometimes doesn’t like to go to church. And it’s a movie in which the title character, an owlish 13-year-old named Austin (Jacob Laval), isn’t merely autistic. He also suffers from osteogenesis imperfecta, which means that his bones are extraordinarily fragile, so that he’s like some teen-geek version of Mr. Glass in “Unbreakable.” Speaking to the audience in voiceover, Austin lists his bone breaks, which number in the high twenties, as if they were Pokémon…