Time for teaching him to be alone. Tire him out with a walk, then put him in the kitchen - leave him with a favorite toy and a chew toy. Walk away - if he howls - turn and say sternly "quiet" then walk away - do it for 5 minutes first and when you come back be sure it is when he is quiet - even if for only a second - praise and let him follow you. Tethering him with his leash to you will also help to keep him out of places he shouldn't go. You can have him on the down/stay in your bedroom for example. Again start slowly and be consistent if you give in he will cry more.
Sting Chance von Gaard AKC GSD 2/8/2006