Neutering is a big help in getting back to good habits. A bunny who is neutered will finally have his hormones settle down after a full month. Often he has a mini-surge of odd behavior just before that, but hang in there.
Once it’s completely over and time passed, he will probably return to normal use of his litter pan on his own. He may do that much sooner after the neuter as well. Most bunnies like a litter pan that is clean to maintain a clean habitat. The don’t really like pee or many poos around, and may even act out until you clean their space again.
If you find that he doesn’t use the litter well post-neuter on his own, use some positive reinforcement. First, put a handful of hay in a nice, big litter box with him so he can eat and go at the same time. Eventually you can use your normal hay rack, but a snack of hay in there will keep him in it longer and remind him what the litter pan is for.
Next is to watch for his tail to be raised and put him in the pan yourself so he remembers that’s where he needs to go. If he pees in it, not outside, give him a small treat (a raisin or a pellet) as a reward and praise him. He will quickly learn good habits again.