While Senator Sanders has said that, his actions and current speech is contrary to his claim.
Eugene Debs on Capitalism
The workers in the mills and factories, in the mines and on the farms and railways never had a party of their own until the Socialist party was organized. They divided their votes between the parties of their masters. They did not realize that they were using their ballots to forge their own fetters.
The infallible test of a political party is the private ownership of the sources of wealth and the means of life. Apply that test to the Republican, Democratic, and Progressive parties and upon that basic, fundamental issue you will find them essentially one and the same. They differ according to the conflicting interests of the privileged classes, but at bottom they are alike and stand for capitalist class rule and working-class slavery.
One question is sufficient to determine the true status of all these parties. Do they want the workers to own the tools they work with, control their own jobs, and secure to themselves the wealth they produce? Certainly not. That is utterly ridiculous and impossible from their point of view. The Republican, Democratic, and Progressive parties all stand for the private ownership by the capitalists of the productive machinery used by the workers, so that the capitalists can continue to filch the wealth produced by the workers.
It is vain to hope for material relief upon the prevailing system of capitalism. All the reforms that are proposed by the three capitalist parties, even if carried out in good faith, would still leave the working class in industrial slavery. The working class will never be emancipated by the grace of the capitalists class, but only by overthrowing that class.
Bernie Sanders on Socialism
”The next time that you hear me attacked as a socialist, like tomorrow, remember this: I don’t believe government should take over the grocery store down the street or own the means of production. But I do believe that the middle class and the working families of this country who produce the wealth of this country deserve a decent standard of living and that their incomes should go up, not down.”
—November 2015 speech at Georgetown University’s Institute of Politics and Public Service.
Senator Sanders does NOT call for the overthrow of Capitalism. His programs are within the framework of Capitalism. Hence, Senator Sanders is not a Socialist.
Note: The nationalism (social/public ownership) of business under Socialism only pertains to large corporations -- usually with a valuation of $10 billion or more. Of course all "commons" will be socially/publicly owned and placed under the democratic control of the workers .
Some examples of the Commons: financial institutions, utilities, communications, all forms of public transportation and related ways, healthcare and education.
 From the Socialist Equality Party: All privately owned industrial, manufacturing and information technology corporations valued at $10 billion or more must be transformed into publicly owned enterprises, with full compensation for small shareholders and guarantees for all pension and health care fund investments. Industries critical to the basic functioning of society—including telecommunications, agriculture, education, health care and transportation—must also be subject to public ownership and democratic control.
This does not mean the nationalization of everything. Small and medium-sized businesses, which employ millions of workers, are themselves in desperate need of reliable sources of credit and relief from the unfair monopolistic practices of the large corporations.