1. The first thing I would ask is do you have older living relatives that may know more about your grandfather?
Do you know the name(s) of any siblings? Aunts/Uncles? Cousins?
Often, you can find additional information through their records.

2. In most cases, you want to examine the record trail from the latest to the earliest.
The latest would be the records created around his death.
a) Death Record
b) Cemetery Record
c) Church Record (?) or Church Program
d) Social Security Notification (where he would appear in the Death Index - SSDI)
e) Bible Record (entry in family Bible?)
f) Real Estate Record, if there was a property sale.
g) Court Record (probate)
- etc. -

a) Do you have a copy of his death record? You should be able to order it from the Division of Vital Records (PA). It MAY have the names of his parents.
With that information, you can trace him (or them) through the census records back to Florida.
- An obituary may list the names of his parents and relatives.

b) His Social Security Application may also list his parents' names.
I suspect he acquired his number from the Social Security Admin, instead of the Railroad Board.
Search the SSDI:
http://ssdi.rootsweb.ancestry.om/cgi-bin/ssdi.cgi

More on SS#s: http://www.rootsweb.ancestry.com/~rwguide/lesson10.htm
(Old link, but good information)

3. Another resource is the U.S. Population Census. If you do not have a subscription to an online census site, then you can find these records, online or on microfilm, at most genealogical sections of local libraries. Your library card may allow you to access them from home.
Also, there are a few FREE sites that have "selected" online census data by state.

a) I found a NELSON Moore in 1930 (Philadelphia).
This MAY be your Nelson Moore.
He was born in Florida in about 1894.
He was living on Federal Street and the proprietor of a Furniture Store.

Listed as separate families in the same house were Peter and Mary Molet from Alabama, Enoch Mason, and Audrey Reed (from GA).

b) Sylvester Moore (26) was in Philadelphia in 1920. He was born in Florida abt 1894 and worked for the Railroad. His wife was 19 year old Bessie Moore from South Carolina.
They lived on the North Side, #2213 Wilder Street.

I found some Moore families in Florida in 1900, including a Nelson (Putnam County), but no Nelson or Sylvester born around 1894 (yet).

In 1900, he would be about 6 or 7 years old and (hopefully) living with a relative or parent.

I can send you a copy of these census records.