Please keep in mind that I've never seen your son personally nor have I reviewed his clinical history in detail. For a more detailed explanation that is specific to his case, please consult with your pediatrician. Having said that...
I certainly can understand your concerns about the anesthesia given his last experience with the prior procedure (which does sound like it could have been a seizure), but theoretically unless a patient is allergic to the anesthetic or has an underlying genetic disease (such as certain muscles diseases) that predisposes them to adverse effects with anesthetic agents, they should be ok undergoing anesthesia even with an underlying seizure disorder. Now there are some anesthetics that can cause seizures such as halothane, but on the flipside, we use other anesthetics like versed to actually treat seizures emergently. Also, the physical/emotional stress of the procedure in itself can reduce the body's threshold for having a seizure, leaving the patient with a seizure disorder vulnerable to having one peri-operatively. The best thing you can do is inform the surgeon (which you've already done) and the anesthesiologist who will be in on the case (if you can, sometimes, they don't know who's doing the case until day of surgery). Also, consult with your pediatrician or peds neurologist for more info. Best of luck.