Thanks for the additional information,
Since he was shaking when he fell over, it sounds like this may have been a seizure. Dogs can have a seizure for unknown causes and it may never happen again, or may begin to happen more frequent or violent. It is always wise to screen a dog with basic lab work (complete blood count and chemistry profile) to make sure that the seizure was not caused from other abnormalities in the body (i. liver or kidney disease, infection, blood sugar issues, etc). I would keep a log of the incident in case it happens again. In the meantime, have him seen by a veterinarian for bloodwork. Having 1 seizure is not enough to put him on medication if the bloodwork is normal, however, if the seizures continue or become more violent (legs paddling, vocalizing, longer duration), he can be placed on anti-seizure medications. As with any medication, there can be side effects and he would need to be monitored. That being said, with frequent or violent seizures, the risk of medication side effect is much less than the side effects from having the seizures.
YOu can read more about seizures and treatment here:
If you are concerned about cost of treatment, here is a link for financing:
Some vet clinics offer a payment plan, for low income or emergency situations.
Hope this helps. Please let me know if you need additional information.