Best practices for someone flying with Epilepsy?

My nephew has epilepsy and has a few seizures a week. His seizures are NOT an emergency, but we've never flown for obvious reasons. But now we need to, in order for him to get a special brain scan in another state. My sister and I will both we traveling with him. She wants to sit in first class so that there is more space, but if he had a seizure the best thing would be to lay him on the floor. But if that happens during taxi/takeoff/landing I know thats an issue - and probably still kind of an issue in other times too.

So does it make more sense to sit in regular seats so the arms go up and if he has a seizure we try to like, lay him across our laps??? He's 15 and about 5'4".

Have any of you dealt with non-emergency seizures on planes before? Does anyone have any advice?