If he had a full-blown seizure, I would immediately take him to a pediatric neurologist. They would be very unlikely to give you a run-around, and would probably see your son quite soon.
We had a similar reaction, only the seizures didn't start until an hour after the vaccines (he got 3, and he was only about 6 pounds at this point), and by that time I was home alone with the baby. I called the pediatrician immediately, but I didn't know it was a seizure (I'd never seen one before), so I didn't SAY it was a seizure, I just tried to describe what was happening. The nurse didn't believe me over the phone, wouldn't let me talk to the pediatrician, and told me that if I brought the baby in, they would refuse to see him because everything was fine, vaccines "don't DO that," and I was obviously over-reacting.
When we finally did get to see the pediatrician, he immediately said that it was a severe reaction, and sent us to a neurologist, who said it was a seizure.
The nurse was fired.
I have found that specialists--neurologists, endocrinologists, dermatologists, rheumatologists etc.--tend to be much more aware of vaccine reactions than pediatricians.
My middle kid's dermatologist told me that they get most of their business from vaccine reactions.
Not exactly reassuring, is it?
Oh, and yes, do report to VAERS (Vaccine Adverse Event Reporting System--https://vaers.hhs.gov/esub/index). Anyone can report--you don't have to be a doctor to report. It's a voluntary system, which means that doctors aren't required to report anything if they don't feel like it, which is absolutely ridiculous, when you think about it.