He wasn't even a social democrat. At the time, social democrats were democratic socialists, the shift away from reformist socialism happened around the 80s (some social democratic parties still hang onto reformist socialism, at least in theory).

He was a smart liberal who realized that in order to save capitalism from collapsing again, some regulations are necessary. In Europe, similar policy was often pushed by social democrats, which sometimes leads to confusion. But actual social democrats at the time went (or at least wanted to go) further, like nationalization and socialization of major industry, worker representation at companies, and increasing worker and union power in general.

Social democrats stated endgoal was a socialist society. FDR's endgoal was to protect and maintain capitalism.

Edit: Also, Bernie is definitely a reformist socialist, I will never understand why people think otherwise. He literally mentions Eugen Debbs, one of the most influencial socialists in American history, as his role model and hero every chance he can.. And he praises the nordic model because the nordic model was literally pushed by reformist democratic socialists.. Here is Olaf Palme, one of the most important figures when it comes to the nordic model and prime minister of Sweden (until he was murdered), explaining why he is a democratic socialist:

https://youtube.com/watch?v=7i2Ws1X5DSA

Just imagine a conservative politican, calling themselves a fascist, keeping a picture of Mussolini on their desk, saying he is their political role model. Would you claim that he isn't really a fascist? It's not even as if Bernie Sanders was dog whistling, he couldn't be any clearer about his believes.. Yet somehow, so many American leftists seem to sonehow doubt his intentions? Why? Because he isn't radical enough? Because he isn't throwing molotov coctails at the police? What does he have to gain from falsely calling himself a socialist??

The man's presidental campaign was giving 20% of major corporations to it's employees and having about half of the board of directors be elected by workers, among other stuff..

if you don't even want to acknowledge his values and his ideology simply because he is playing the politics game and is a reformist, send him to Europe, we would love a genuine leftist like him with so much charisma. I don't think you appreciate him..

Imagine dedicating your life to fight for a better life, involve yourself in the civil rights movement, work in various socialist groups, calling yourself a socialist and calling for major industry to be socialised, being constantly attacked by right wingers for your socialist believes, etc, only for fellow leftists denying that you are a "real socialist"..