How often is he being crated? He's associating being crated with something he doesn't like, probably boredom.
I would try making going into the crate a really positive experience: toss a few treats in there after he obeys your crate command, and make sure there are safe and durable toys and bones inside. If he's going to be crated for awhile, make sure he gets plenty of exercise and is tired for the crate! If he gets crated for longer periods daily due to a job etc I'd consider daycare or a dog walker a few times a week to keep him active and mix up the schedule!!
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