I had the very same experience with my 2 months old puppy. She was so cheerful the day and by night she started to seizure and i quickly took a spoon not to let her mouth close. Using dropper or syringe, i dropped little water on her mouth. I then gave her .5 ml of liquid paracetamol of humans as that was the medicine i have for fever. After few minutes she starts to walk though weak. I kept her warmth and was with her till sunrise. She is now four months old and very hyper and jealous...
I am SO SORRY for your loss! I really don't know what it could have been, other than from your description, and from the fact that this was a young puppy, it was something congenital that wasn't discovered by the vets who examined him and gave him his puppy shots. That's not unheard of, and it certainly is no reflection on the abilities of the vets, since there are so many things it could have been. There is any number of things it COULD have been, but the only way to tell for sure is to have a necropsy done. The fact that you said he appeared to be having a seizure makes me think it was something to do with the brain, and there is no way this would have shown up in a routine puppy examination. I would certainly let the breeder know what happened, because if they are a good breeder, unless it was something that happened as a result of your negligence (which I don't see how something like this could be, unless the puppy got into something toxic in your home), they should replace the puppy for you. It's also a good idea to let them know because if it was a congenital defect, depending upon what it was, they need to know so that they don't repeat that breeding (re-breed the same two dogs) and possibly produce more puppies with the same defect.
Ghilly