Z.Z.
Oh, I'm so sorry he has to go through surgery. It's always hard on the parents, but know that God is holding your little one in His hands (and holding you, too).
As for food for him, you'll have to ask the doctors. If his diet is restricted it will come from the hospital kitchen. If anything goes after surgery, take the foods from home that he likes.
Take things to comfort him.
Things to keep him busy.
Something new.
Soft things in case he throws them.
Nothing too noisy. There are other kids around trying to sleep.
For yourself, reading and hobby stuff. Your own pillow and blanket. Small bills for vending machines and parking.
Deodorant, wash cloths, towels, soap, nail clippers, hair brush, shampoo, flip flops for shower, comfy clothes, toothbrush and paste, make-up to feel pretty, glasses/contacts (take both if you wear them), phone numbers, writing material, envelopes, stamps, if you plan to write out of staters. Laptop if available. Kids sleep a lot. Laptop charger. Cell phone and charger. Gum and mints. Fem hygiene products if there's any chance.....
HTH.
Z.