One of the authors changed idea before submitting paper
Talk to the other collaborators who probably agree with you, and then take a unified front to tell him that holding several collaborators hostage is not ok.
Additionally, if part of his new publication uses a jointly developed idea which he intends to publish alone while blocking your joint work (that should be in the references of his planned solo paper), then this strikes me as borderline plagiarism. In any case, it’s very bad form.