First of all I have absolutely no idea what is wrong with Jack, but I agree with Karen. He needs to see another vet and quickly! Don't be fooled into thinking that he is OK because he occasionally perks up. My dog, Ashley, was severely ill with liver problems, couldn't keep anything down and was listless, etc. but when someone would come to the door she was there with her tail wagging. Dogs are just different than we are. Even when facing horrible illnesses they somehow find the ability to do things that give us reason to be encouraged. I am so sorry to hear about Jack and will be following this thread. I do hope there will be a diagnosis quickly and one that can lead to his healing. (((hugs)))