The film ends with a close-up of his face. This breaks the illusion of the fictional narrative, giving the film a greater sociological purpose - the problem of the boy's delinquency. We have to provide an effect. This notion subscribes to principles of Bertolt Brecht , that sympathy sometimes just isn't enough. Having no effect leaves the problem with the audience, who, in terms of fictional narratives, expect everything to be 'nicely tied up' by the end.
Prefiguration prepares the viewer for what is going to happen.
Although the ending of Les Quatre Cents Coups is placed in our hands, we have prior knowledge that the boy is a truant.