"... I've checked Union & Wanderer's Rest in Milwaukee. He's not there. What would be the oldest Lutheran Church in the area that he might have belonged to in 1850 or 1860 ... Was St. John's around back then and if so where can or how can I get in touch with the sexton of their cemetery? ..."
Try St. John Ev. Luth. in Milwaukee (8th & Vliet St.). They founded a cemetery that pre-dates Milwaukee's Union cemetery by 5-10 years. Then 2 other Luth. churches joined with St. John's (Milw.'s first recorded Lutheran miracle) in a business venture to create Union Cemetery.
St. John's original property is the southern end of today's Union cem. On a Union cem. map, it is the first section (family plots?), and probably also the 2nd section (individual burials?).
It would be best to contact that church for info. on their early burials. As the city expanded, the same cemetery corporation created Graceland cem. and their office has the Union cem. records.
The Union cem. office does not have St. John Ev. Luth. Church earliest burial records, and the office staff are not historians. Frankly, I don't believe they ever had the plot records that St. John would've sold by subscription to their church members. The advertisements for St. John's cemetery plot subscription drive (1850s) and the later Union cemetery plot subscription drive (1860s) were published in Milwaukee's German newspapers.
And with all of that, I think St. John's parishioner burials may refer to "St. John's Cem." for several years, even when Union Cem. was the official name. (Based on what I see in the German papers.)
"Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950).
Gary Rebholz, compiler/editor
Indexing Milwaukee's German newspapers continuously since 2007...