Diapering
What we typically do is use diapers (huggies #1 or some other brand of size 1) to keep the bun dry. I pull the diaper, still folded, from the package, and cut a hole out of the butt end of it. When you unfold it, it is in exactly the right spot for the tail to poke out. I like this because the tail stays dry and also the poops come out, which prevents them from mixing with the urine and getting all mushy. Then I usually lay the rabbit on a nice synthetic sheepskin rug wherever he wants to be. Your rabbit may be mobile with his front legs, in which case he'll just pull himself around your house with those. My last guy, Dandelion, was super strong with his front legs and could move very fast. So I just let him do what he wanted, and changed his diaper twice or three times a day.


Feeding
In terms of feeding, if your bun's appetite is still good, then he can eat anything he wants, as much as he wants. You want to provide food for him ALL the time, to keep his weight up (he'll lose quite a bit just from a loss of muscle mass). If his appetite is not very good, you can syringe feed him a pellet slurry (recipe in House Rabbit Handbook) and also baby food carrots, squash, applesauce, and stuff like that (step 1). Sometimes when they first get paralyzed the appetite goes away, and if you syringe feed him, it'll start to come back bit by bit till he's eating 100% on his own. That's what happened with Dandelion. So don't be too discouraged if at first he doesn't want to eat or drink on his own.
Keeping Spirits Up
Beyond that, just give him lots of attention, and make sure he doesn't feel isolated.