Q. Why is my dog shaking and stiff ass if he is having a seizers
A. That may be exactly what is happening to him. Make sure he is in a safe place where he can't hurt himself, away from furniture or anything that can fall on him. If he seems to be recovering, call your veterinarian and let them know what happened. Keep a close eye on him, if he has another seizure on the heels of this one, then he needs to be seen by your vet right away. Sustained or repeated seizures can cause the body temperature to rise to dangerous levels and increases the risk of brain damage.
When you have a chance, take a look around and see if you can find anything he might have gotten into, since poisoning is one of the causes of seizures. If he has a history of liver or kidney disease, this is also a potential cause. Sometimes we never know why a pet has a seizure. If he does have a series of seizures, your vet will want to do an exam and bloodwork to try and determine a cause and potentially put your pet on anti-seizure medication to lessen the chances of this happening again in the future.