Can you post some photo of his setup? You could add some shelves or logs perhaps so he would need to jump on or go under when he wants to reach his food or toilet, just place them on his way so he can't avoid them. I had a rescued rabbit, still have him, I thought he was very old he was absolutely helpless couldn't jump on anything I thought he has arthritis or something, I made a shelf for him with a lower step, then removed this step so after a few weeks he learned to do it and with this easy exercise he became more active day by day. Also I agree that some rabbits are more active than others and that 3 weeks is maybe just he's not completely feeling safe in your house especially if he's a free-roam or his area is too roomy and uncovered, maybe he is feeling safe under your bed so you can make a few hides for him or make his official residence under your bed just move his stuff and perhaps playpen closer to it.
