I don't think so if it was post convention and Hillary had already won it.
If it was pre-conventionI, I think Bernie would need a majority of the delegates at the convention to vote for him. Seems to me he would be likely to get it, but it would not be automatic. Its theoretically possible I suppose that he would not be able to get a majority and that eventually another candidate would emerge...but I doubt that the party is suicidal enough to snub the Bernie voters like that, but then there was the railroading in the Nevada primary so maybe...
Wouldn’t be a HOOT if the Dems are the ones who have a brokered convention?