Have you tried giving him a shoebox house?
If the house is large and dark inside they nearly always move in because they like nesting in a dark area. You just cut the base out of a cardboard shoe box and keep the lid as a lift off roof (so you can check inside) Then cut a hole for a door I cut an arch door for older hamsters in case they struggling lifting their legs to climb through a hole). If you make the door at the far end of one of the long sides of the box, then the other end is dark (light doesn't go round corners). They alwasy nest at the dark end. And it's big enough you can put a corner litter tray inside at the door end at the back. All my syrians have done this - nested at the dark end and they create an ensuite toilet separate from the nest and use a litter tray if you put one in. It makes life easier as then you just lift the roof off to empty the litter tray once or twice a week and the rest of the cage stays clean and dry.
It might be easier if he's getting older as it would mean less clean outs needed and his nest would stay in tact. They tend to build a really big cosy nest in a large house. It might be easier for the walking if he's struggling with that a bit. It means you can keep the substrate firmly pressed down for a level surface to walk on and a level route into his house. If you have a bendy stick bridge to put over the door, that makes it even better as it's even darker inside then and a tunnel entrance tempts them to move in. Plus it doubles up as a ramp onto the house roof, so it's like a very low shelf.
I just put a big pile of toilet paper strips next to the house (not inside it) and they take some and build a nest inside. If you put a smelly treat inside the house (like cucumber or cheese) it's likely to tempt him in. Once they find a large dark space that mimics a burrow they prefer it I think
Just a thought and it might make his life nicer and easier in his old age and less cleanouts to bother him.
