It’s great that he goes to the litterbox to pee and that he’s eaten a little bit.
He’s not too old to be neutered, but the very young recuperate a lot quicker from a neuter. What you describe reminds me of my Bam’s neuter. He was at least 2 years old, probably more, and he didn’t feel well post neuter. I gave him a hot water bottle that he snuggled up to. Rabbits can easily get hypothermic post anestesia, so if you have a hot water bottle or handwarmers, I’d give him that. There must be enough room for him to move away from the water bottle if he doesn’t want the heat though.
I had to syringe feed Bam the day after his surgery, because he ate only very little. I used pellet mash – ie pellets soaked in water. To perk the bunny up a bit you can add something sweet to the pellet mash, like baby fruit puree or mashed banana. The sugar in the fruit goes directly out into the blood and that can help a lethargic bun feel a lot better quickly and hopefully well enough to start eating of his own.
Fresh greens and herbs are often the most popular food post surgery.