Hi,
I am sorry to say, this does sound like a seizure.
Seizures can occur for no apparent reason. This type of seizure activity is referred to by some vets as primary epilepsy, or idiopathic epilepsy. Most of the time the onset of seizures in dogs with primary epilepsy is between one and five years of age and there usually is a fairly long interval between the first seizure and subsequent seizures when they occur.
Seizures can occur as a reaction to medication, allergies, toxins, other diseases, fevers and anything else that disturbs brain function. These seizures are sometimes referred to as reactive seizures or secondary seizures. It is often possible to figure out the cause of this type of seizure based on the history of another illness known to lead to seizure activity, the clinical signs at the time of seizuring or a known history of using a medication that may lead to seizure activity. Allergies are a lot harder to rule out as a cause of seizures, especially food allergies.
Since this was the first episode and you are unsure of the cause I would strongly recommend speaking with your vet and having a full workup done to see if they can determine the cause and begin any needed treatment. You can wait until the morning if he has recovered and has no other attacks tonight.
I am including a link where you can learn more about the seizures and what to do in case of another one.
Walter
Breeder
Breeder and pet health consultant. Have spent a number of years with Behavior Training &health care.