You can notify USCIS by calling or including a letter with your filing so that paperwork is mailed to you, not your former attorney. I suggest you hire a competent immigration attorney always, however.
He doesn't send a letter. He sends a G-28. If he didn't file any paperwork then he shouldn't have sent a G-28..
My answering this question does not form an attorney-client relationship. Always retain a qualified attorney before taking any action. My office offers free consultations.
Assuming he sent notification that he was representing you (which would be odd if he never filed anything), you can send a cover letter with your petition telling CIS that you are representing yourself and to mail all correspondence to you. .
Samuel Ouya Maina, Esq. 415.391.6612 email@example.com Law Offices of S. Ouya Maina, PC 332 Pine Street, Suite 707 San Francisco, CA 94104