Here is a good article on seizures that may help. There are a number of things that can cause seizures. It would be best to have your dog evaluated by a vet. Do you know the people that owned your dog previously? If you could contact them maybe they would let you know if he has had these before (if they are honest). Please keep us updated as to what you find out. He will be in our prayers.
http://www.thepetcenter.com/gen/epilepsy.html