I would do your best at redirecting that behavior. My sister has a dog that has broken multiple teeth due to biting at the crate wires. Hopefully this is just an 'I'm a puppy and I have to put my mouth on everything it passes' type of situation. Theo (at 9 months) still does some of that mouthing on anything that he passes, but it has gotten much less frequent over time. Maybe you could put a yummy Kong on each side of the crate to distract him from the door. I've never had a real need to use Bitter Apple Spray or something of the like, but maybe that would help as well.