George Slawson book has your Great grand father listed as the son of George Washington & Rebecca ( Storms ) Slawson
This is what is listed in the book...
Horace Henry Slawson Born Aug. 17. 1897 at Queensboro , Orange Co , NYMarried Dec. 28 1921 at Warwick Ny Grace Anna DeGraw, Born Feb 16 1900 sister of Sarah Jane DeGraw,
and daughter of Ricard & Mary Elizabeth ( Benjamin) DeGraw. Resided at Sterling Mines, NY moved to Fort Montgomery , NY : Moved to Cornwall,NY : moved to Highland Mills ,NY where they resided in June 1944
Hope it helps find some new information.
Note: The book does contain mistakes But I found most information correct .