From looking at your photos his fur loss looks very similar to my Syrians who was diagnosed with cushings earlier this year. This is caused by hormonal imbalance. I do recommend seeing a vet preferably an exotics specialist if you can find one near to you as their is a treatment option - vetoryl which can manage the condition. This thread by Coco61 is really useful for cushings and has some picture so you can compare better with your Syrian
Cushings and Treatment (my vets didn’t do blood tests as her symptoms were obvious enough)
Mites can also cause fur loss too but that usually is alongside irritated skin and often you notice your hamster scratching more. A vet again would be best to see as they can have a better look at the skin.
Is your hamster black? Hard to tell from the photos but black hamster as they get older their fur lightens and turns more brown - browning. So I wouldn’t be concerned with the change of colour.