My son had this done in july. It's a sleep deprived EEG, and they are probably testing him for epilepsy right?
They want him to be sleep deprived because they want him to take a nap during some part of the EEG, because certain epilepsy types are triggered by lack of sleep.
My son has benign Rolandic, which is triggered by lack of sleep, he managed to sleep a little during the EEG.
He was not sedated, and as far as I could tell they don't usually sedate during an EEG.
I will warn you (and you can tell your son, or leave him in blissful ignorance until you get there) that is hurts like billyo to have the EEG electrodes put on the scalp. They use some sort of abrasive cream on a Q tip to rub off the surface of the scalp, then put a dab of glue on, then the (38) electrodes. hurts.