Were the bites hard? Apart from biting out of aggression/being territorial, rabbits do bite to communicate and groom. If they’re more like light nips and doesn’t seem like he’s biting you to hurt you he may be nipping you for attention or to communicate things like “I want more food” or “hey hooman, you’re in my way!” Sully bites to communicate, he’ll usually nudge me and lightly nip if I’m in his way or if I’m standing up and he comes along wanting pets. He’ll also bite when ‘grooming’ me. Not so much if he’s licking my skin, but if he’s doing it on my hair or clothes he will since rabbits tug on fur when grooming.
Sully did have to be taught to bite lightly though. When I first got him he didn’t understand what pressure was acceptable. But I taught him to go lightly by squealing when it was too hard. Every now and then I have to remind him though.
If your buns doing it for the above reasons, squealing and jumping/moving away should work to teach him how much is okay. But if they are quite painful and seems to be with the intent to hurt that may be for aggressive or territorial reasons. It’s good practice to avoid intruding in pens as much as possible since that’s their safe space and they can be territorial about it. I clean Sully’s litter and pen when he’s out roaming. If you’re still litter training him, you’re space isn’t ready for him to explore, or he’s not ready/able to be let out of his cage like that maybe try moving him to a new space so he doesn’t feel like you’re invading his safe space? It might take some time, but the best way to stop it also is paying attention to what’s going on when he does it and figure out what he’s trying to communicate, be it anger or something else. (As you already seem to be doing from what you said. It can be hard to figure out what they’re trying to tell us and can take some time.)
Either way, even if it is aggression squealing and such should train him to stop the habit. Or at the very least get him to use more reasonable nips.
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