As long as he's been medically cleared, I'd probably not bring it up with the airline. What could they do to prepare that you haven't done already? By bringing it up, you risk them denying/delaying your flight for "medical reasons."

The frequency of seizures is something you'll have to consider when you decide to fly. Are we talking every couple of hours, or every couple of days, and if several times a day, what are the chances it will occur during flight? Depending on the seizures, do you have to deal with bowel/bladder dysfunction, and is that taken care of? None of these questions needs to be answered here, and you likely have already considered them.

Best wishes for your husband, and I hope things go as well as possible for him.