If you're keeping him confined to his pen while he recovers from his neuter, it could be he's doing it out of frustration or boredom from being confined. You'll need to determine if he's trying to move the box and that's causing the mess, or if he's digging it out.
If he's trying to move it, you can use clips or ties to secure it to the pen bars. If he's digging it out, you may need a larger litter box with higher sides. Plastic storage bins or cat litter boxes are common ones used. If that still doesn't solve the issue, for my rabbits I made a screen to cover the litter and prevent digging. Plastic cross stitch canvas is one of the easier ways to do this. I used plastic window screening for mine. Look over those litter training links I shared in your other thread. There's also this person's story of their journey solving a similar issue.
https://mylittlebunbun.com/fix-rabbit-digging-litter-box/
Though this may solve the digging issue, you'll still be left with a restless bun. So it would be good to add some enrichment activities. A separate dig box is an option, boxes and paper to shred (only if bun doesn't try and consume much of the paper ripped up, GI blockage hazard), rabbit safe grass or wood 'toys', etc.
https://wabbitwiki.com/wiki/Toys_and_games