Aloha! You're speaking to Dr. Michael Salkin
I'm sorry that your question wasn't answered in a timely manner. In general heat exhaustion is considered when there's a history of environmental exposure or recent strenuous physical activity such as hunting, chasing, or seizures
. Panting is certainly consistent with heat exposure but stiffening of the legs isn't expected unless seizures, metaldehyde or mycotoxin intoxication, or hypocalcemia exists and except for seizures, those latter etiologies are unlikely. Collapse rather than stiffening is expected with heat exhaustion.
I'm concerned that he suffered a partial seizure (previously called petit mal and now also called psychomotor seizure) which is defined as focal or asymmetric sensory or motor activity affecting any part of the body and which can be associated with autonomic signs such as drooling and vomiting. Seizures arising at such a young age are usually due to idiopathic (unknown cause) epilepsy
. We'll accept one mild seizure monthly before considering anticonvulsive medications.
Please have his vet perform a thorough physical and neurologic exam plus biochemical testing in the form of blood and urine tests. Yorkies are overrepresented with hepatic shunts (abnormality in the vasculature in and around the liver) which can predispose to seizure activity. These dogs are usually quite ill, however, which doesn't appear to be the case here.
Please respond with further questions or concerns if you wish.