Have you checked the 1860 and 1870 censuses to see if you can find a first name? If he died in Pittsburgh from 1870 on, then there should be a death record at the Register of Wills office, but they will need to know the first name in order to do a search:
Allegheny County Register of Wills
City-County Bldg., 1st Flr.
414 Grant St.
Pittsburgh, PA 15219
412-350-4180
www.county.allegheny.pa.us/regwills/cert.asp

You can order a copy directly from them. You could also try the Carnegie Library, which has an index to obituaries and death notices. Less comprehensive than the death records, but worth a try.