Hi there. This is a problem with the smaller houses - they do need to build a large nest for the winter - its instinctive and also practical as they feel the cold
The bigger the house, the happier he'll be but don't put it on the shelf, put it at floor level on top of the substrate and a house that's open underneath. That way he can burrow down a bit and bury hoards under his nest and it stays cleaner and better ventilated.
A cardboard house at floor level is a good idea - a shoebox is ideal. Cut the base out of the shoebox and keep the lid as a lift off roof (although don't keep taking the roof off as that freaks them out - it's just for the rare occasion you need to check or empty a litter tray).
If you cut a door at one end of one of the long sides of the box, he will nest at the opposite end where it's darker - and you could even fit a corner litter tray inside at the door end at the back - if there's enough room. They tend to use the litter tray if it's inside the house. If the house isn't big enough for one don't worry, he'll probably choose a corner of the cage as a toilet and you can put a litter tray there.
To tempt him into the new house you could put a smelly treat inside - eg cucumber or cheese and a bit of hamster food. And put a pile of paper nesting material next to the house or nearby (but not inside it - they like to take it in themselves to build the nest).
Ideally put that in without taking the old igloo out and wait for him to move in - they usually move into a big dark house overnight - or maybe the second night. But if he's very attached to the igloo (had that before) you might need to take it out altogether if he doesn't move into the cardboard house straight away.
But be careful until he moves as they can go into torpor if they get too cold and that usually leads to death. If he doesn't have enough nesting material at the moment he could get very cold at night - so until he's moved and built himself a big nest, make sure the room is kept warm enough - 20 degrees. Overnight when the heating is off the temperature can drop very low and it's coming to the time of year when we get frosts overnight.
I have a small oil filled radiator I leave on in the room overnight - on a thermostat - so it only comes on if the temperature drops below 20 degrees - which at this time of year it does at night so it does come on, but goes off again once the temperature is warm enough.
They can also get a bit upset and anxious at their nest being removed or not secure.
If you have a bendy stick bridge it's also a good idea to put that over the entrance of the cardboard house to make a tunnel entrance - it doubles up as a ramp onto the flat roof. That almost always tempts them inside!