I don't think Bernie will ever really drop out. He means it when he says we need a revolution, he's from that school which thinks it's better to have someone like Trump than Hillary because it will bring the revolution faster. Same reason some people on the far left hate unions, because they see that as a tool that enables the system to work, which postpones the revolution, because without unions the workers would probably revolt.
We'll see, but my feeling is he'll have his mind on seeing Hillary lose, because if he plays his cards right he won't be blamed for that (it will be blamed on Hillary's unfavorables or scandals), and if that happens it means Trump becomes president and people will inevitably rally around their "defender". His movement would gain massively, and he'd be leading a big shakeup in the Democrat party that would be very difficult to put down, and in that context probably rightly so.