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.