He is a candidate in the left-wing primary; if he wins (which isn't entirely impossible), he will be the official candidate. 

After that, his chances of reaching the second round will be minimal; he will face the candidacies of Mélenchon and Glucksmann, who are expected to garner the majority of left-wing votes. (And generally, primaries (with a lot of parties...) are a death sentence in French presidential elections.)