Stemmer in austrian states in phonebook:
Vorarlberg (41)
Oberösterreich (23)
Steiermark (20)
Niederösterreich (19)
Wien (16)
Tirol (6)
Burgenland (2)
Salzburg (1)
http://www.herold.at/telefonbuch/stemmer/
outside Vienna possible a place in Lower-Austria/Niederösterreich nearby:
http://www.herold.at/telefonbuch/nieder%C3%B6sterreich/stemm...

Vienna/Wien-area: http://maps.google.at/maps?hl=de&q=Vienna&bav=on.2,o...

so in fact Stemmer are living "outside Vienna" north and north-east of it. so maybe possibilities for relatives then would be (about Stemmer-persons in phonebook):
2232 Deutsch-Wagram
2231 Strasshof an der Nordbahn
2103 Langenzersdorf

a point to start maybe would be to look at online-files in the above stated places if his possible birth is to find. civil-records started in Austria nearly 1938, before to research at parish-church (or/and diocesan-archive).
http://www.matricula-online.eu/?lang=en
possible dioceses:
take >>>
for Langenzersdorf > AT-DSP St. Pölten, rk Diözese / rk diocese
for Deutschwagram and Strasshof an der Nordbahn > pla..t: AT-W Wien, rk Erzdiözese / rk archdiocese

if this is too difficult for you you can take a professionell researcher - if you want to pay money for help: www.ihff.at

for asking directly at parish-church at places where Stemmer are living today:
Deutschwagram: http://www.erzdioezese-wien.at/edw/organisation/vikariate/de...
Strasshof an der Nordbahn: http://www.erzdioezese-wien.at/edw/organisation/vikariate/de...
Langenzersdorf: http://www.pfarre-langenzersdorf.at/content/aktuelles/