One possibility that his feet got frozen, but I guess we can exclude that, right.

I'm no vet, and I wouldn't know too much about clinical things, but my guess it's from an injury of some sort. If the finger bones were broken, it's likely that the tendons have pulled the fingers together and they healed in that way (for instance, bird's feet without muscle tension, e.g. in a dead bird will curl up like that). Maybe it was from the car impact, but it could also be some previous injury.

I hope this has been of some help.