Your description sounds like epilepsy. My former dog had grand mal seizures. We had to give him phenobarbital and another drug which aided in the absorption of the phenobarb. He was a lot slower, but he lived another 5-6 years after his first seizure.
When your dog falls over, does his body become stiff with an arched back? Do his legs flail like he is running? Does he urinate?
My dog’s seizures came on suddenly when he was 7 or 8 years old. There was no prior indication of any medical situation. One the vet had his medication balanced, he would only have one or two seizures a year.
There is nothing wrong in seeking a second opinion from another vet. Sometimes another doctor will see something the first missed. If nothing else, the other vet can confirm the diagnosis.
” When your dog falls over, does his body become stiff with an arched back? Do his legs flail like he is running? Does he urinate? “
Yes , somewhat stiff , but no arched back . He urinated the time I set off the cap gun , but not the following two times .