Dogs bark for various reasons. They bark when they are bored frustrated, lonely, when they feel threatened etc. Try to understand the reason for the dog barking, maybe something simple like providing him with a toy or something to occupy him will stop his barking. Somethimes dogs get into the habit of barking, and do it for no reason other than that they can. There are methods to train a dog to stop barking, which teach the dog that barking is not acceptable behaviour. However in order to use these methods you must be around when he barks. If he barks when nobody is around, I suggest that you look into other methods of training, like anti barking collars (not the kind that give electric shocks - those are very inhumane).