If your client does not qualify for expungement, his remaining option is a pardon from the governor. If the governor gives a full, unconditional pardon, the effect is to remove all the legal effects of the conviction. But, to seal the record, your client needs to qualify for record sealing.
Ohio Revised Code Chapter 2967 contains the laws regarding pardons.
The application form and instructions are on the Ohio Department of Rehabilitation and Correction's Clemency page.