It will end up to one of these three:
Either Bernie bolts and goes indy, his supporters vote for him that way, or other two possible choices would be, support Trump or stay home or at the dorm.
Option 4: Vote Hillary
She’ll probably end up with 80%+ of his voters who actually intended to vote.
They are socialists and Hillary remains their best bet. Most will get over their butt hurt and vote for her, but many who have to go to some effort to register, absentee ballot, etc. won’t bother.