I'm not related - based purely on circumstantial evidence, I think his parents may be Henry Lamar b. ~1828 TN and Mary Moore b. MO.They are enumerated in Ray Co. MO. Appears that Mary Moore Lamar probably died at the birth of William or shortly thereafter, as she is not shown in the 1870 census. I think Henry Lamar married Mary Fields after his 1st wife's death.
(Based on birth month of October (1870 & 1900) and proximity in the state of Missouri.Henry & Mary's son John Lamar is living with a cousin of his mother's in Caldwell Co. MO in 1880.Caldwell is between Daviess and Ray Counties.)