Hello and thank you for using this site to help you. There are a couple of things that can cause what you are seeing. Drooling can be due to nausea, or a tooth or gum issue. Shaking can be from fear or pain.
I would check his mouth for tooth or gum discoloration (yellow, black, brown, red) and feel around for sores. If you notice this , it may be a good idea to have the teeth checked specifically by your Vet. Teeth can also fracture below the gum line which can cause pain and you can't actually see that, you may need an x ray to see that.
There are seizures that are not as violent as Grand Mal, but these seizures alter the dog's behavior. Often times you might see some staring off, some air licking, lip smacking, acting scared and either distant or over clingy to the owner, or snapping at imaginary bugs. Just so you know for future reference.
I think you are dealing with a pain or nausea issue. It could be back pain ( a disc along the spine) , a tooth issue, or a stomach issue such as pancreatitis.
Your best bet if it happens again is to bring him in for an exam for these specific symptoms. Likely when you last brought him in it was for overall vitals and general exam.