He could actually be suffering with depression saw in dogs
They love to please their companions and if you aren't stimulating him or her with walks, exersize, games, attention etc.,then it might be simple boredom. If they can't burn off the inherent energy, you may get yourself a dog with too much time with nothing to do and then they might have to find it on their own accord and that could be chewing, barking, spinning. A heck a lot of bad behavior is manifested due to them being bored outta their minds.
If a dog is sad then that means they aren't getting any companionship ! You may try to play with the dog or bring another dog to cheer him or her! But be fast, dogs don't look good sad;)
He might be sick.
Or maybe he needs a new toy to play with.
Maybe he needs a day at the doggie park.
If condition persists for more than two weeks, consult your local animal expert. Hope he feels better soon.