(assume he says "I am making no decisions right now" after CA, but after DC...)
"At this time, given the results of the primaries, I acknowledge I cannot win the nomination through normal means and that it would be incredibly undemocratic for the superdelegates to give it to me. I concede this nomination to Hilllary Clinton. However, I cannot endorse her for president and will not campaign for her. Ultimately, Hillary stands against the american people on fundamental issues and is a puppet for other wealthy people to use at their will. I cannot in good conscience support that. I will be voting for a third party candidate. I ask my supporters to use good judgment when choosing whom to vote for and not make blind choices based on some stupid idea of having to prove loyalty to the DNC chairwoman."
I would hope Bernie would never utter such filth about our dear Hillary, but if he were then it's off to the guillotine with the traitor!
If Bernie actually did this, he'd have a Democratic opponent in his next Senate race. And he'd lose, to either a Democrat or a Republican.