At 18 months of age, he is right at the beginning of the prime age for idiopathic epilepsy to appear. It usually affects dogs for the first time between the ages of 18 months to about 3 years.
It usually takes a while for them to get used to the effects of the anti-seizure medication, and until they do, they do appear to be very drowsy and out of it. Your dog should adjust to his medication within about a month. If he doesn't, you should talk to your vet about adjusting his dosage. There are several medications that are popularly prescribed for idiopathic epilepsy, so if you can't find a workable dosage of phenobartibal, you might want to ask your vet about either Dilantin, Valium, potassium bromide, felbamate, or a different type of anti-seizure medicine that can be used in conjunction with potassium bromide and/or phenobarbital together called Zonisamide. Zonisamide is totally unrelated to any of the other anticonvulsants, but, like Keppra, the full extent of its side effects is not yet known so to some vets, it is somewhat controversial to prescribe it. Dilantin, Valium, potassium bromide and felbamate, along with phenobarbital, are all tried and true when it comes to treating idiopathic epilepsy. The main thing (and the hardest part) is being patient while you wait for your dog to adjust his body to his dosage so that he can learn to live normally and not be sleepy all the time.