I’m so very sorry this happened.
The best thing you can do for your bun now is to try and spend as much time as you can with him. You dont necessarily have to interact with him all the time, but he should be in the same room as you as much as possible. Keep a close eye on his appetite and poop, but I’m sure you’re doing that anyway because of the surgery.
His first night home you should make sure he eats something before you go to bed for the night. Wet greens or fragrant herbs are often the first thing they want to eat post surgery. If he doesn’t eat on his own, you can syringe feed him Critical Care or pellet mush (pellets soaked in water).
He might benefit from a warm water bottle that he can snuggle up to if he wants it, but he must have enough space so he can move away from it if he doesn’t want extra heat.
Also serve water in a bowl if he normally has a bottle he might not feel up to bothering with a bottle.
It’s good if you can sleep in the same room as he at least this first night.
Again I’m really sorry for your loss. It’s so sad and so unfair.