This is all assuming that he is fixed and a change of hormones isn’t to blame:
Just like you said, it sounds like acting out to me! He’s been naughty in other areas, which is his way of sending you a message. He’s just saying it with poo, too!
I say this b/c when my bunny poops outside his litterbox, it’s generally for a reason. I rearranged the furniture, and he was not pleased. He thumped a lot and left me a pile of poop in the middle of the floor. I took him to the vet. He wasn’t pleased. He left a pile of poop right next to the litter box. So CJ has said it with poo in the past.
I’m not sure what else to tell you, but I think I’d try to figure out why he’d be upset. I doubt it has to do with how often you change the litter. If he is out a lot, you might try confining him again until it gets under control. Sounds mean, but that’s how you litter train to begin with– start in a small space and work up to larger ones. It helped me when training to immediately take care of the pellets that don’t make it in the litter box by scooping them into it ASAP.