[Maker]

From what you have explained there may be some easy fixes.
- don’t rotate his toys, they get very upset when something is taken away. So set up his cage and don’t change it
- he has a large cage and lots of bedding so stop cleaning it - he will feel like he is going into a new environment every time you do. Poo’s don’t smell and you will need to put up with the wee smell for two weeks after which you can just clean the wee’d on bits - not a massive clean though, it still needs to smell like his. Their eyesight isn’t great and smell is very important.
Don’t touch him for 2 weeks, just put his fresh food in and water every day, do it quietly with no fuss.
Let him get used to you sharing his room and the family sounds.
I would reduce the amount of walnuts to one every 2weeks, maybe a few flax seeds or a piece of rice on the other days - just for health reasons.

Can you perhaps attach a photo of his set up - does he have a large wheel, is his house a large bottomless one? So he can keep a food horde in there under ground as well as make a cozy nest for himself? You can scatter shredded toilet paper in his cage for him to use.

Once he starts to feel safe and secure he will respond better to you but the key is to stop doing anything for a while then restart Really slowly.