Based on what?
What he calls himself. We throw labels around like "democratic socialism," and we pretend that it's more moderate than totalitarian socialism. It isn't. They both advocate the public ownership of the means of production and view capitalism as something to be eliminated. What most ignorant people call "democratic socialism" is actually social democracy, which is still basically capitalistic but with regulations and a generous social welfare system.
The difference is much more visible here where people aren't locked into a two-party system. For example, in Germany, the SPD (Social Democratic Party) is, as the name suggests, as social democratic party. It's a center-left party that basically supports free enterprise with a lot of welfare and labor protections - much like the Nordic model. Die Linke (The Left) calls itself democratic socialist, and they're the successor party to the Socialist Unity Party that governed East Germany. They're pretty much of a bunch of capitalist-hating crazy people who claim to support free elections (since overt totalitarian communism is somewhat politically taboo here).
Bernie Sanders isn't an idiot spouting his mouth off. He knows the difference and the nuances, and he chooses to call himself a democratic socialist.