[Maximo]

I would start with your regular vet and basic blood tests and exam.

If he is trembling every time he sits down, that doesn't sound neurological. It could be something minor like anal glands, or it could be something serious like an injury. I would see the vet sooner than later.

Relative to seizures, keep in mind that not all seizures symptoms are alike. Sometimes it looks like the dog is focused, eyes fixed forward on something. However, if you were to squeak a toy near the dog, the dog would not take note of it.