It could be nerves and just the excitement of traveling. My mare gets it right before a bit storm or when a cold front blows through. Not all the time but it’s just the weather change that bugs her.
I like the idea gina_pasquini has about wrapping his legs. The polos will be messy, but it might work! And try a whitening shampoo. Those work the best on white coats. I like Mane and Tail the best =) Good luck!
~If you come at it having only 15 minuets it will take all day... If you come at it having all day it will take you 15 minuets~