“Beautiful Boy” has a lot to recommend it. It develops real characters. It treats them with generosity, and offers a rare portrayal of an amicable divorce, in which the spouses still care but are no longer in love. Unlike Gus Van Sant’s “Elephant” and reportedly unlike Lynne Ramsey’s upcoming “We Need to Talk about Kevin,” it doesn’t aestheticize school violence. Debut director Shawn Ku seems more interested in a sensitive portrait of the minutia of suffering.