Take out every hop, ramp and climb
A rabbit with stiff joints still wants to reach his litter tray, hay, and favourite sleeping corner under his own power, but each hop up into a high-sided tray or climb to a shelf costs pain he will start avoiding. Adapt the home by making it honestly flat: one level only, no ramps to food or shelter, and no raised platforms presented as enrichment. If his hutch has an upper floor, close it off and move everything he needs below.
Cut down one side of the litter tray to a low walk-in entry, or swap to a large low-sided tray, and fill it with a soft layer of hay over litter so landing is cushioned. Place hay in a pile beside the tray at floor level rather than in a rack he must stretch up to, because stiff necks and sore backs make reaching up as hard as hopping up.
Give his feet grip and his joints cushioning
Slippery laminate, tiles, and bare hutch wood force a stiff rabbit to splay his legs to stay upright, which strains hips and shoulders with every step. Cover his whole living and exercise area with a continuous non-slip surface: low-pile rugs, interlocking foam mats, or fleece over padding, laid edge to edge so there is no slick gap between his hide and his tray. Check that rugs do not ruffle into ridges he must clamber over.
Warmth matters more as movement slows. Site the pen away from draughts at floor level, give him a deep bed of hay or a padded hide on the ground where he can stretch out fully, and in colder rooms add a pet-safe heat pad under part of the bedding so he can choose warmth without being confined to it. A rabbit who can lie stretched and warm will often move more willingly afterwards.
Arrange the pen around the route he still uses
Lay out tray, water bowl, hay, and hide in a compact loop with wide straight runs between them and no narrow gateways, tunnels, or furniture legs to negotiate. Use a heavy water bowl on the floor as well as any bottle, since a stiff neck may make bottle drinking awkward, and refresh both where he already pauses rather than teaching a new corner late in life.
If he lives with a bonded companion, watch that the younger or fitter rabbit is not monopolising the warmest hide or nudging him off the best hay pile, and add a second hide and hay station if needed. Gentle space to move still matters: keep a clear run long enough for short, voluntary wanders, because complete confinement stiffens a rabbit further even though jumping and climbing are now off the menu.
When housing changes are not enough
Better housing should bring back reliable tray use, steadier eating, and more time stretched out in the open. If he still misses the tray, drags a hind leg, falls to one side, stops eating hay, grinds his teeth while resting, or has a soiled underside he cannot clean, pain or another illness is likely involved and needs a rabbit-aware veterinarian, not another rearrangement. Ask promptly rather than assuming old age explains every change.
With KinsLe Family Sharing, you can share his profile with the family members you choose, and a relative topping up his hay opens that profile rather than a retelling of who he is.