What Causes Seizures In Young Adults?

My 19 yo son has fainted/passed out with shaking like seizures, gets all clamy, and white. This has happened on three visits to the dentist. After the first time, we shrugged it off. The second time we went to see a doctor to follow up. They suggested he may be allergic to litacaine or similar. The dentist told me they used something different today... and he still had an incident. This time it was a little more scary. Should I do another follow up with doctor to see what is causing this... or is my son just a fainter?