Based on his clothing, I'm guessing the man is a Civil War veteran. And the buildings in the background look military, too. The photo was probably taken in the 1880s.
Snapshots of Civil War era dogs are rare. Sources point to the popularity of the big breeds during that time. My sister, Barbara Brackman, posted something on her blog about dogs in (not on) civil war era quilts. Read more at her Civil War blog.