One black balloon is visible in one short shot, but it’s a free-floating signifier of nothing in writer-director Elissa Down’s sunny, slight first feature. The rest of this coming-of-ager is legible to a fault. Thomas (Rhys Wakefield) is a new kid in town going to a new school and making no new friends. Except Jackie (Gemma Ward, a model with a lunar mien) who helps him on his swim stroke in phys. ed. and sneaks a kiss while practicing CPR. His dad (Erik Thomson) is in the military, which is why the family moves a lot. He gives Thomas his first driving lesson. But the big lesson in “The Black Balloon” is brotherly love. How to love his big autistic brother Charlie (Luke Ford), who’s all id and a real chore to mind. The family locks the front door to keep him from doing things like running down the street in his underwear and going into stranger’s houses to use their toilets. It’s sooooo embarrassing when it’s the house of Jackie and Jackie is in the shower at the time. On his birthday, Thomas hits a breaking point and beats the crap out of his Charlie. This cues the predictable breakthrough into unconditional acceptance of disability and all its downsides. Best part is Toni Collette (“Japanese Story”) as the boys’ mom. Worst is the chirpy, pesky music by Michael Yezerski. Down draws upon her experiences growing up with two autistic brothers. No idea what in the past of her co-writer caused him call himself Jimmy The Exploder, which White Stripes fans know is the title of one of their songs, though not obviously one about an Aussie screenwriter. 97m. (Bill Stamets)