Given these clues, I would say that the answer has to be that the Scotch half breed was a messenger.

In the book, the Scotch half breed was carrying mail to Dawson.  We are told that the dogs were pulling a heavy load because it was the "mail train."  However, the word "mailman" only has seven letters.  The words "mail carrier" have 10 between them.  "Mail train does have 9 letters and it actually appears in the book, but it's not the man's job.  So all I can think of is to say that the man was a messenger, which has nine letters and does make some sense given that letters are messages.