I would take him to the vet to rule out some dangerous causes of seizures as soon as they're open. My mosaic chinchilla however, periodically has seizures, and they are usually brought on by stress in his case. His body gets kind of rigid, and gets unsteady on his feet. When it happens, I take him out and hold him until it is over, and he is fine afterwards. However, I started looking into what would stress him out, and stopped exposing him to those things, and he hasn't had one in over a year now.
Regardless though, there are some seizures that are caused by dietary deficiencies, and other things, so I would take him to the vet to rule out those things. Be ready to tell the vet what kind of food/treats you are giving him, as well as any other dietary information they may ask for. Get him checked out at the vet to be sure.