Well, I'm afraid that if you've responded to him (fairly regularly, it seems) when he howls in his crate then he *knows* that that is the way to obtain release or some attention. It would take a LONG time of not responding to break him of the habit (dog's memories aren't that short).
The first thing I wonder, given that this is happening overnight, is whether or not he's howling to go to potty? What do you do with him currently when you get up? *If* it's a potty thing, you might like to have a vet check him over, and if there's nothing wrong, think about limiting his water intake in the evenings.
If it's got nothing to do with potty needs, then your options are as follows:
(1) Ignore the howling. Consistently, absolutely and for however long it takes for him to be convinced that that technique no longer gets your attention or him released from the crate.
(2) Move his crate into your bedroom. Being near you through the night may well be sufficient to stop the behaviour.
(3) Stop crating him overnight, and instead give him a dog bed either in your room or elsewhere in the house where he's likely to be content until morning. He's probably old enough by now to be trusted loose overnight.
Good luck
