A neuter is a fairly light procedure, so there’s not much you need to do in preparation except see to it that he’s eaten (never fast a bunny).
Clean the cage, of course, if there are more than one level, block it off (no climbing or jumping the first 1-2 days after the procedure).
Put shredded newspaper in the litter-box instead of litter that can stick to the incisions (which will be small and glued shut).
It’s good to have Critical Care and feeding-syringes on hand in case the bun won’t want to eat after – but that practically never happens with a young boy, it can happen with an older boy though.
A heat source like a snuggle safe or a hot water bottle is sometimes appreciated – buns can get hypothermic after having ben anaestetized. Again this rarely happens with young boys.
Provide water in an open bowl instead of a bottle for easier access (bunny will be a bit groogy from the anaestetic).