It is very difficult, because rabbits have a deep urge to pee and poop-mark beds and sofas – soft comfy place with lots of human scent on them. This is rather more of a strong urge and a very natural thing for a rabbit to do than a choice. He does understand that you disapprove – but perhaps not what it is you disapprove off. Basically, I think he could very well go on peeing and pooping on the bed when he can, just not when you are there to see him do it. Dogs respond better to human disapproval, but dogs have been genetically adapted to humans through selective breeding for at least 13 000 years – rabbits haven’t gone through that kind of radical genetic changes through their domestication process at all. Rabbits are more like cats, they have retained a lot more of their natural behavior and needs.
I think I’d try blocking off the bed or covering it with a shower-curtain when you’re not there. You can place a litterbox or puppy pee pads on the bed and hope he goes there.
As for neutering, it helped with my bunny Bam who was marking the sofa, but there is no guarantee that it helps. Many spayed or neutered buns still mark beds and sofas. Those are places of high value and thus important for a rabbit to mark.