I'm sure the OP has learned this is not the way to transport a dog. If not, it will happen again. His vet probably gave him a serious dope slap -- they hate this sort of thing.
Here it is illegal to have a dog improperly secured in the back of a truck. If you must transport in the bed of a truck, they make specific cross ties for this. They should only be used with a harness, not a collar as your dog should never be secured in a vehicle with anything but a harness.
I have two trucks - one a standard cab without airbags. I set up a platform for the passenger side & secure the dogs with seatbelts. The other is a crew cab. Dogs ride in the backseat secured with seatbelts because there are airbags in the front.
I also have three sets of cross ties. If I'm traveling I try to have one of those set up for securing any stray I may find and need to transport. If I transport in dog boxes, those boxes are secured with ratchet tie downs.
Dogs loose in truck beds = pet peeve.