Triggers for seizures vary dog to dog. This could be a metabolic issue such as thyroid disease, it might be a sign of liver or kidney disease too. Idiopathic seizures sometimes don't show up until the dog is right around this age.
Use of heartworm or flea medications or other drugs can cause seizures in susceptible dogs.
This site really goes over seizures and causes well and you may find it really useful
http://www.canine-epilepsy-guardian-angels.com/site_map.htm
You may find phenobarbitol will help reduce or resolve the seizure activity.
If you know his breeder you should talk to them about this and see if others in the litter or either of the parents have the same problem.
Hope this helps you!
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