Phenobarb is for control of brain based seizures- makes seizures less severe and not last as long. It is NOT fast acting- takes a while to reach steady state. Valium is for treatment of the actual seizure. Phenobarbital does not address muscle based tremors like due to low calcium directly- but...
Did the vet check his blood sugar? Or calcium? Epilepsy is a seizure based in the brain- more or less an electrical disturbance which the phenobarbital would suppress, but there are other causes of seizures or seizure like muscle tremors- like low blood calcium, low blood sugar...