Why did he called a referendum in the first place? It seems a bit of a silly topic to call a referendum about, IMO. Can't the government change the law only by legislative manners?
Tecnically yes, but this matter is a self-inflicted wound of this government. One of the promises of AMLO was putting on the ballot the charging of former presidents as a demagogic policy but it's a poisoned chalice because, firstly, you can't let the people decide the law. Second, if you let people decide the law why bother have a congress, they can decide economic and social policies and that's a Pandora box by itself.
In fact, it's quite easy in this case. Presidents after their term is over don't have immunity, so they can be investigated and charged if any illegal activities are proven like any other ordinary citizen. Presidents in Mexico after leaving office do lose immunity, right?
Because he’s a populist who loves “consulting the people” because he thinks it gives him more legitimacy.
In this case, I'm with Socani, this is a self-inflicted wound by AMLO.