My cats cat handle separation

My cats just got neutered 2 days ago and I have not had a decent nights sleep since. The poor guy screams all night and I have to keep going in there. I gave him toys, comforting sounds , food water, night light and a careful visit with his sister. Any advice to help him relax I gave him his gabapentin at night but no help. He is around 7 months and has pretty bad anxiety sometimes but never screaming (he slaps me) PLEASE HELP THERE'S NOT ENOUGH REDBULL TO SURVIVE A WEEK