Start by watching the routes, not the rooms
A cat with stiff joints rarely complains; he quietly edits his life. The windowsill visits stop, the top of the cat tree is abandoned, he sleeps on the floor side of the bed he used to share, and he may hesitate, circle and then rush at a jump he once made casually. Walk his day with fresh eyes before changing anything: the route to the litter box, to food and water, to his sleeping place and to you. Every jump, slick stretch and high threshold on those routes is a candidate fix, while obstacles in rooms he no longer enters can wait.
This order matters because it spends effort where his pain actually lives. A ramp to a chair he ignores helps nobody; a single step beside the bed he still tries to reach every night changes his life daily.
Take the height out of favourite places
For the places he still loves, lower the cost of arrival. A stable pet step or a carpeted ramp with a gentle slope lets him walk up to a sofa, bed or wide windowsill instead of launching from the floor and landing hard on sore front legs. Anchor whatever you add so it cannot shift under him, and give the top a surface with grip. Some cats take to a ramp immediately; others need a few treats placed along it over several days before they trust it, which is normal rather than stubbornness.
Apply the same thinking in reverse to litter access, the change owners most often delay. A high-sided box that a young cat bounded into becomes a daily hurdle, and some stiff cats start toileting beside the box rather than fail at the climb in front of an audience. Swap to a box with one genuinely low entry side, or cut and smooth a lowered doorway in the existing box, and place it on the floor where he now spends his time rather than at the end of a stair flight.
Grip, light and shorter distances
Slipping does quiet damage to a stiff cat's confidence. On wood, tile or laminate, splayed hind legs during a startled scramble can leave him sore and reluctant to cross that floor again. Washable runners along his main routes, a rug under the feeding station and mats where he lands from any remaining low jump give him purchase. Keep his claws trimmed, because overlong claws change how the paw meets the ground and worsen the skating effect on hard floors.
Then shorten the map. Gather the essentials, box, water, food, bed and a scratching option, on one level, ideally within one or two adjoining rooms, so comfort never depends on stairs. A low night light on the litter route helps an older cat whose eyesight is also fading, and slightly raised food and water bowls can spare a stiff neck and shoulders a deep bend at every meal.
When home changes are not enough
Adaptations buy comfort, not a cure, so keep watching the cat rather than the furniture. Book a vet visit if he stops attempting a route he previously managed with help, cries out when picked up or stroked along the back, hides for long stretches, grooms one joint obsessively, or his appetite and litter habits change alongside the stiffness. A vet can assess pain directly and discuss whether more support is appropriate, and your notes on which changes helped, and which he ignored, sharpen that conversation considerably.
Once the new layout is in place, record where everything now lives in KinsLe through Family Sharing, so family and helpers follow the same routes and details instead of rearranging his world back.