This does not answer your question of his parents, however if you look closely at the image of the 1850 Freeport census - his name appears to be Orestes not Custer, and his son William O named a child Orestes H.
850 Census Freeport, Ilhttps://familysearch.org/ark:/61903/1:1:M853-W7R - see image, first name is Orestes
IL Marriages
Citing this Record: "Illinois, County Marriages, 1810-1934," index, FamilySearch (https://familysearch.org/ark:/61903/1:1:KF2S-R29), Orestes H. Wright and Mary M. Atkinson, 15 Jan 1838; citing Kane, Illinois, United States, county offices, Illinois; FHL microfilm .
Name:,Orestes H. Wright
Marriage :15 Jan 1838 Kane, Illinois
Spouse: Mary M. Atkinson
Reference ID: item 1 122
lIlinois Deaths and Stillbirths son of William O and Mary E (Humphrey) Wright
Citing this Record: "Illinois Deaths and Stillbirths, 1916-1947," index, FamilySearch (https://familysearch.org/ark:/61903/1:1:N3TD-98T ), Wm. O. Wright in entry for Mr. Orestes H. Wright, 16 Oct 1934; Public Board of Health, Archives, Springfield; FHL microfilm 1,712,220.
Name: Mr. Orestes H. Wright
Death:16 Oct 1934 Event Place:,Rockford, Winnebago, Illinois
Male Age 60
Birth Date:,12 Jan 1874 Freeport, Illinois
Father : Wm. O. Wright Birthplace:Freeport, Illinois
Mother: Mary E. Humphey Birthplace:Hartford, Conn
Occupation:,Farmer
Residence :Lancaster Township, Stephenson County, Illinois
Spouse:,Alice Dorman
Burial Date:18 Oct 1934 Burial Place:Florence Township, Stephenson, Illinois
Cemetery:,Oakland