Hi there. He is gorgeous! Hair loss can be caused by a number of things, and at this time of year, when it's warm, he could be just having a moult - losing his winter coat sort of. I didn't know about this before, but our hamster had this at about the same age last summer (see photos below). Got very mangy looking, It all grew back and I started giving him a pinch of linseeds daily as well (works wonders for their fur) which he really loves and licks them all up each day. Very healthy! You can get a big bag for not very much and they last forever. Brown linseeds are the best.
There are other things that can cause fur loss. And Cushings disease is one - but that is hard to diagnose. Coco on here has a vet in London who was able to do a blood test, to test for it, but no vets I have met are able to do that or know how to (hamster's veins are teeny tiny). There is a urine test but it's very unreliable.
So personally, as he looks healthy, I would assume it is a moult and give him some linseeds. If it gets worse and doesn't start growing back by the end of summer you might want to have some further investigations. Cushings disease isn't very common. Other symptoms of that are sores and bloating, and weak back legs.
This photo only shows a little bit but it looked worse!
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