Shelley (p. 280) includes "Shepard, Negro" in the list of "Revolutionary War Soldiers, Sailors and Patriots"
"Massachusetts soldiers and sailors of the Revolutionary War" has an entry for Shepard as a private in Capt. Samuel Sayer's company, serving two months in 1779 on the Penobscot expedition
Bibliography:
My Name is Wells - by Hope M. Shelley (2002)
Massachusetts soldiers and sailors of the Revolutionary War - Office of the Secretary of State (1896)