Our dog is exactly the same he is 5 , its called seperation anxiety, we have tried to cure it with the many "cures" on the internet and it has partially worked, , personally we have found that a squirt in the face with some water stops him in his tracks.... thankfully we have good neighbours who are old and deaf...best of luck
Leave a radio on when you go out or better still put a tape of your voice on. Also you could try going out for a couple of minutes then come back, try this several times leaving it longer each time, it takes a lot of time and patience but I've known it to work.
We had the same problem as well and still do to an extent - we're lucky our neighbours have noisy dogs as well.
We tried something from the vet .called a DAP - Dog Appeasment Pheromones. It's a plug-in thing, like a Glade air freshener, that releases the pheromones of a lactating bitch, which is supposed to remind them of when they were puppies and calm them down.
Make sure he has got lots of things to keep him occupied, a knuckle bone or pigs ears, or maybe a treat ball that he has to work to get the treats out of. Also a good run before you leave him should help to make him tired and sleep while you are out.
Just a thought - does he actually howl all the time you are out?
Mine do howl just after I have left the house, but its only for about 20 - 30 seconds and then they settle down. Try taping him to see how long he does it for, or wait outside for a couple of hours to see how long it goes on. It may not be as bad as you think.