Silverfish, I am so sorry about Jack .
Your situation is extremely similiar to mine...although in my case the vet wishes to send him for a lot of tests and procedures and my houndie, who is 4 months shy of his 14th birthday has white coat syndrome in a big way.
Just knowing he is going in the vehicle...or thinking the vehicle, or more specifically I might be going somewhere without him is enough 'excitement ' to launch an attack.
He will settle in the vehicle...at the vets he does not, it escalates and is terrifying.
At home he is calm, no attacks unless he is too warm, eats too fast, or gets too stressed about going out.
He sleeps quietly, has a good appetite.
Getting an xray will be difficult, let alone into the vet office and he could stress himself fatally in the process
His worst attack to date was begun as we headed 2 mins away to the vet for a routine nail trim in a quiet, no other patients , end of hours vet clinic . 6:45- 9 pm it lasted. He was home 1/2 hour when it ceased.