Herbert's Ohio death certificate shows he was born March 6, 1884. A WWI draft card for Herbert Stclair (that is how it is indexed) born on the exact same day shows his nearest relative was Charles E. St Clair of Rochester, PA. The 1910 census shows Charles 34 with a wife Hattie and sons Charles & Herbert. I believe Hattie or Harriet was widowed by Charles by 1930.
Perhaps by finding something out about Charles it will lead you to info on Herbert. Check with the New Castle Public Library and Lawrence County Historical Society to see if they can help you. Hopefully Charles' obit will list Herbert.
If he served in the military the National Archives should have his service record which may have some useful data.
Any clues about Herbert's background can be quite helpful. Did he have any siblings or other relatives that the family knows of?