Are you sure about that?
I think Maryland, Connecticut, Pennsylvania etc are all closed primaries. If he loses them, it's over, well before he gets to California.
A closed primary isn't an issue if they allow same-day voter registration...
The problem comes when you have a state like New York... which requires you to register 6 months before the election... before Bernie's campaign even started!
I understand New York has had those rules for forty years - and a 74 year old like Bernie should have known about it, given that the rules have been in place half his life.
He lost because people just don't like him. The ones who did like him were put off by his stumbling answers in his interviews to the New york press. He's done.