Dog Howls in Kennel When He's Left Alone - What Can We Try to Help??

Do you mean an indoor kennel or an outdoor kennel?
 
The way dogs learn is very context specific so to become accustomed to being in the kennel when you are in the house doesn't automatically translate to being OK with it when you leave the house. 
 
Have you tried leaving him in there, getting you shoes and coat on etc.... and then taking them off again, calmly letting him out of the kennel as if nothing happened and rewarding him for remaining quiet?  Then progress to going outside and coming back in, walking down the street and coming back, getting in the car, driving round the block and coming back.....gradually leaving him for longer each time. 
 
How much exercise does he get before you leave him?  What other stimulation does he get before you leave him -training, play, etc.  Clicker training is tiring for dogs.  Aim to get him quite tired before you leave so he learns to use "alone time" to rest.
 
Make sure you ignore him completely 15mins or so before you leave and do the same when you get home.  Downplay your comings and goings so that he is not in a highly emotional state when you are out.
 
Other things you can do to help soothe him:

• leave a radio on quietly for him
  
• leave a t shirt with your scent on
  
• a dap diffuser/spray
  
• rescue remedy in his drinking water
  
• leave him with something like a well stuffed kong to keep him stimulated and occupied
  
• leave him plenty of OK and safe things to chew on (helps relieve stress)

buy your dog a Tri-Tronics no bark collar. Let  him wear it while you are gone but be sure to remove it when you are home because the points might irritate his skin. Your neighbors will thank you!

Does the Tri-Tronics colar work for howling and loud whining, or is it effective only for barking?
 
Thanks!

I don't know but I would only use it as a last resort.  Barking is a natural activity for dogs and I can't imagine what it does to them to get a shock each time they do it.  Try other training methods first and explain to your neighbours you're in the process of fixing the problem and stopping the barking.

I agree that the no-bark collar should only be a LAST resort, and one I would personally never use!  With a shock collar you are not treating the problem, only the symptom.  You are not accomplishing anything but harming your dog.  Take chuffy's advice!!

It sounds like he could have some separation anxiety.  I would do a search for that here on the forum -- there should be a ton of information about SA you can read. 
 
I would *not* use a shock collar.  It is likely to increase the dog's anxiety and that would be a really bad idea with an SA dog.  Certainly, do not try a shock collar until  you have exhausted all the non-cruel ideas for behavioral treatment of the problem. 

Have you tried leaving him alone uncrated in a confined doggy safe area? How long and often do you leave him?