Hi Dina it is very worrying when you dog experiences a seizure episode .it would be a good idea to take him to the vet and get checked as there are all types of reasons for seizures and this will help to rule out any other factors.
Try looking at this web site on epilepsy as it has some good info on there,
Canine Epilepsy Website
Try and keep a diary of when he does have these episodes as it is useful to have it written to keep a note and you may be able to to see a pattern and what triggers it off.