Oh I absolutely think dogs can get sad. i believe they have the capacity for a full range of emotions. Disruptions in routines and changes affecting their pack members are the most likely thing to set off an emotional reaction in a dog, outside of a physical ailment to themselves. They definitely experience grief and loss, so why not sadness?
Sounds like your routine -- in Charlie's perspective -- changed because of your surgery and subsequent different behavior, i.e. having less time and attention for him.
Like others have said, give him extra play, love, exercise & attention. Doggies are so attuned to us! You can't underestimate that.
