
When I worked at a boarding kennels although they did have insurance it did not cover illnesses, clients paid vet bills on their return. I'm not sure how a broken leg would be viewed but if it was a freak accident and not due to negligence at the kennels then I would say the bill is your responsibility, either to claim your insurance or pay.
Accidents can happen all the time, hounds break legs playing, bouncing around. I know several that have broken limbs running on flat grass with absolutely no bunny holes or anything. I would have a good look around the kennel, run and exercise area where your boy was kept, if you see something of concern that is clearly dangerous then of course you have a claim, if not I'd say it was just one of those things.
I hope your boy heals well anyway.