Your dog is probably bored and lonely.
Give him stuff to do while you are away.
Here is a tip I copied off this site...it may help
"At the very least, provide your dogs with good toys to amuse themselves. Many toys (Like Kongs) can be stuffed with peanut butter or other dog treats. This gives the dog something appropriate to do when you are not around. Another tip is to keep the television or the radio on. Talk radio is thought to be better than music and is soothing to most dogs. Nothing though, can replace human interaction, not even a second dog, which is not a good idea if you don't even have time for the first one."