Novacaine has Been off the market for years, due to a high precent of people being allergic. Thus being said, I doubt the local your son was given was novacaine, probably lidocaine.
Local anesthetics can cause seizures, I think some of the local anesthetic must have been injected inadvertently into an artery. Usually very high doses are needed to produce a seizure but if it is injected directly into an artery going into the head and neck area much smaller doses are required.
it just might be from the epinephrine in it. He could be sensitive to it. I am and they say that it is a normal reaction but I still don't like it. Tell them you want your son to have carbocaine or anything else without the epi. If you decide to go to another dentist tell them what happened to him and say you don't want him to have it.
This is only somewhat related to the deep concern you have for your child.
I had a lidocaine (?) injection which was one of dozens over the years.
One such numbing up caused me heart palpitations and my face went white. The doctor gave me oxygen to breathe while I got over it. I really can't say why that one injection caused that reaction after having so many many over the years.