I'm going to be a little more emphatic than the other two answers: no, an attorney cannot say that he represents you after you have fired him, so long as the attorney knows that you've fired him. Based on the facts that you've described, he knows that you've fired him and thus it is unethical for him to tell anyone that he still represents you. Furthermore, as a general rule, and subject to only limited exceptions, an attorney is not ethically permitted to refuse to provide information to his client.
Finally, the assignment he asked you to sign sounds fishy, although I haven't researched the ethics rules on assignments, but I can say that, to the extent this assignment is intended to be compensation to the attorney, an attorney cannot ask for or receive a nonrefundable retainer - an attorney is always required to return that portion of a fee that is either unearned or unreasonable.
I would agree with the other answers that you absolutely need to find another attorney as soon as possible to preserve your rights against GM and to get competent advice on what to do about this attorney. I would also suggest that you discuss with a new attorney whether you should file a disciplinary complaint against this attorney with the disciplinary committee for the judicial department covering the place where this attorney has his offices. Many people, and attorneys, are wary about filing disciplinary complaints against an attorney, but unless attorneys who engage in obviously bad behaviour are reported, they will continue to abuse other people.
I hope you hired a new lawyer before you fired this one. If you did, your new lawyer should know how to handle this problem. If you didn't hire a new lawyer, you need one. You can't go up against GM without a very good lawyer. It also requires very specific expertise to deal with these attorney fee lien issues. They can get very messy.
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