I can't give you any information about where your relative might be buried, but I can give you some further background. According to the Michigan Adjutant General's 1905 work, "Record of Service of Michigan Volunteers in the Civil War," he did die in Smithfield, N.C. From the movements of the 19th Michigan, it is likely that he took sick in March or April and was left at that location which probably had some sort of field hospital. By the time he died in mid-May, his regiment was already 30 miles north of Richmond, Va. on its way to Washington, D.C. via Alexandria. Your GGGrandfather appears to have been the last fatality in the 19th Michigan.
He would have been buried in Smithfield unless his family made arrangements to have the body sent home.