How much does he weigh? Valium and phenobarbital are drugs with side effects and damaging to the liver, but if the seizures are bad than you might not have a choice. Did they perform a blood test to see if anything looked strange there, like high ammonia levels because seizures are also a symptom of liver shunt. What type of seizures does he have (see link below for epilepsy info and it describes the different types of seizures. Does he have any other symptoms of a liver shunt?
Liver shunt symptoms:
incoordination
sporadic weakness
disorientation
head-pressing
behavioral changes
drooling
stupor
mental dullness
anorexia
pacing
vomiting
diarrhea
blindness
seizures
coma
Do the seizures correspond with the monthly heartworm meds?
Here's a natural approach to epilepsy.
http://www.nzymes.com/Testimonials/b...OVMTC=advanced
Here's another link about diagnosing and treating epilepsy and all the tests that should have been performed to diagnose it as such.
http://www.canine-epilepsy.com/FAQ.html#anchor6015685
Read through the links I provided and do some research on your own and you will be better able to help your vet and your dog.