Hi William, His parents names should be on his death certificate. Contact the Oregon Archives, arcweb.sos.state.or.us/county/cpclatsop/death.html . They have Clatsop County Births and Deaths 1915-21, 2 volumes.
If that doesn't work out, do you know when and where Leonard Hilton was born? If I can find him and his family in the Census, that should help prove that Hiram was his child.