Wikipedia also has him listed as "Dubose Heywood," as do numerous other pieces of sheet music, websites, etc.   
http://www.google.com/search?hl=en&q=%22Dubose+Heywood%22&btnG=Google+Search

I'm not saying you're wrong, just that I've been seeing it printed as "Heywood" in reputable sources for decades. Now you've got me curious.

However, if Enc. Britannica and his original book covers have it as "Heyward," I'm inclined to believe that somehow there's a misspelling of his name (as "Heywood") that's become very widely disseminated.