You may have already collected his death notice from the local German newspaper. If not, they frequently infer a birth date by the deceased's living years, months, days (like a civil death certificate). If the death notices references the church/Pastor, that will lead you to those records. His name is listed with about 60 other Zellmer entries in "Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950). Some other surnames found referenced with Zellmer are Diedrich Muckerheide Rodau Ulrich Weber Wolske Schureg Marquardt Schultz.
Gary Rebholz, compiler/editor
Indexing Milwaukee's German newspapers continuously since 2007... helping to access our historic German-American culture.