That is a lovely big enclosure. Where does he nest? Might I suggest adding some more floor toys. That gives them more variety as well and makes life a bit more interesting. I fine the large cork logs are popular (the ones that are open underneath). It’s something to run through but also to climb over with an interesting texture. Because they are large they are also a kind of overhead cover - so they have something to sit under without feeling exposed.
Do you know if he is actually using his wheel? It looks about 8-10”. Is that right? If they don’t use their wheel and don’t get enough activity this can lead to weight gain. It’s difficult to see the wheel size from the photo but sometimes they prefer a larger one and use it more if it’s easier to run in.
Understand that sometimes we need to clean the actual cage - it’s a good idea to not replace all of the substrate though - even if it’s just a sprinkling of clean old stuff on top so it smells familiar.
Also - are those pine shavings? It’s not a criticism and you maybe already know that wood shavings need to be kiln dried and dust extracted (and state what wood it is on the packet).
Hamsters do tend to hoard and even nest in tubes - even large ones. Because it’s an enclosed space and they like to feel secure. Our Syrian has a large rat tube along the back of his cage. He doesn’t exactly hoard on it (his hoard is under his nest which is usual) but he does put special things in it! Like a special chew or treat.
If you add anything to enrich his life, I would suggest a multi chamber hide. There is space for it and they love them as they’re dark inside. It also helps with normal behaviours - they will pee in a different room to the one they nest in and use a litter tray.
You maybe already have one - I can’t tell from the photo.
Please don’t take any of this as criticism - just suggestions
. They more enrichment at floor level they have, the more active they are.
It is a worry if you think something is wrong with them and there may be something mouldy in there.