What will sometimes work quite weell is to give him a shot of water from a hose. If you have him in a kennel or run you may be able to hook the hose set with it set on spray that will cover the run. Now when he starts to bark slip out and turn on the spigot to give him a squirt. It has worked on two dogs that I had. It should take no more than a couple times to stop it. Good luck!