Dear Mr. Foskaris,
Thank you for posting your request on History Hub!
Name changes are normally processed by state courts. We suggest that you contact the courts in the county where he was living. See the Illinois Judiciary Court Information by County page to locate the court.
There may be a record for him in the Numerical Identification Files (NUMIDENT), created, 1936 - 2007, documenting the period 1936 - 2007 that is available through the National Archives Access to Archival Databases (AAD).
You may wish to contact the Social Security Administration for questions about historical policies and any records still in their custody.
We hope this is helpful. Best of luck with your family research!