I'm sorry you are having issues with your pet. If you got your pet from a breeder please inform them as they may be interested in your case. Reputable breeders will remove chinchillas from breeding if they produce epileptic pets since it is a genetic problem. Unfortunately seizures can be caused by other things as well.
Did your vet do any blood work? Could it be an issue with diabetes, calcium or electrolyte deficiency, heart or kidney function? Usually seizures have underlying causes along those lines but not always.
Do you notice any patterns or connections between his behavior to try to find what triggers it?
What diet are you feeding?
What is its environmental temperature?
Has your chinchillas ever been exposed to high temperatures over 80F for prolonged periods or have had extreme heat stress in the past? This can cause brain damage that can cause seizures.
Has your chinchilla ever been dropped or received an injury to its head?
With epilepsy the only thing you can do is make sure he has a cool and restful place to go to. (See following post). The seizures will make him very hot and he will have to cool down before he gets back up once the seizure is over. Don't do anything rash just make sure his environment is normally 73F or below.