Since learned that this fellow is thought to be a Warren Tapley, Assistant Surgeon of the 8th Massachusetts, and that the photo was taken in early 1861 in Elkridge, Maryland. Sorry I did not have that information when I originally posted.
Hope y'all enjoyed it.
Edited with an afterthought: I'm now wondering what the earliest photograph is of someone with a letter with an identifiable (pretty much anyway) stamp.
I'm late to this party but I thought I'd drop an idea no one else brought up. Civil war era photographers were notorious for staging photos. The pipe and letter could well be photographers props rendering dates irrelevant.
I'm not obsessive enough to do it but if you looked through lots of Brady photos you might find many different soldiers reading the same "letter from home". Too bad we can't read the address. if it was M. Brady we could be sure.