Thank you for the additional information
Seizures can be caused by many different things.
Problems with the liver or kidneys, Diabetes, Hypothyroid, or toxic ingestion, Epilepsy or may in some cases be idiopathic(meaning having no known cause).
Unfortunately Epilepsy is a common health concern in this breed and this can develop between the ages of 6 months and 5 years most commonly seen between the ages of 2 and 4.
The sleepiness you saw before the actual seizure may have been what we call the pre-seizure stage.
It is also common to see some lethargy or extended sleeping after the seizure. Most dogs recover in a few hours however some dogs can take as long as 24 hours to fully recover.
Here is additional information on seizures and Epilepsy
It would be best to have him seen by your vet, they will want to do blood work to check for underlining health conditions. If all blood work is normal your vet may simply advice monitoring him. Often no treatment is recommended unless another seizure occurs and they become more frequent.
Treatment for seizure can include phenobarbital and in some cases potassium bromide is used in conjunction with the phenobarbital.
I hope this has been helpful,please reply back if you have additional questions or concerns