The vet may make sure he’s eating before you leave (mine did) but give him 24 hours before he shows any normal eating pattern. He will want to eat something, but just provide some greens, hay and his pellets (carrots or a favorite like a banana slice if he isn’t eating other food) and water so he can choose which and how much. Often it’s the medication effecting his appetite as well as the procedure. As long as he eats something. If he refuses, call your vet. Ask for a dose of pain meds to take home with you the day of the procedure in case he is in pain later when it wears off. He may sleep a lot.
In fact, do not hesitate to call your vet once he’s home with any and all questions. It’s part of the service they provide, and a fast phone call can relieve you of worry and keep you on track with him. Meanwhile, I would try to remove the 2nd level from his current house, or you might temporarily attach a NIC cube to block it off. Do you have an XPen? If so, he could go into that or have that open onto his house so he can roam farther. In a few days he should be able to run around the room if he doesn’t jump up on couches or chairs.
You want to check his incision site for swelling, redness or oozing. You can hold a treat above his nose so he stands and get a peek without holding him. Or lay a mirror on the floor under him to see it.