It's difficult to know as hair loss can be caused by a number of things, including ageing. Although with ageing it tends to start behind the ears in my experience.
On his photos he does appear to look a bit rounded/swollen at the back end (wider). Have you noticed any swelling in his abdomen area?
Ideally you would need a vet check. It's unusual for hamsters to bite unless they are in pain or scared, but he may have had some traumatic bad handling experience before you got him.
Sometimes we need to handle them to check them so wearing gloves can help if he bites. (Eg thick ones like leather or gardening gloves) You will be more relaxed if not fearing a bit and so be able to handle him more gently. But do it over something soft/low down in case he wriggles free and falls.
There's some info on hamster hair loss below. They can actually have a moult in warm weather as well. Our Syrian looked very much like yours in the summer of his first year and it turned out to be a moult and all grew back again.
To help with fur condition, linseeds are hugely beneficial. Natural oils and vitamin E. I gave a pinch of these a day and our hamster's fur all grew back - it can really give them good fur condition ongoing. Not expensive to buy a bag of linseeds and they last for a very long time (over a year or more). I gave them separate to the normal food on a separate little dish. They like them and just lick them up.
It can take a few weeks but can really help. Aside from that though you need to know what is going on. It may be nothing serious, but he is getting to the 18 month age that is considered old. If he seems well in himself then it could be ageing or diet or moulting. What hamster mix does he have?
But he does need a gentle checkover really if you can find some gloves - to see if his abdomen is swollen - as he may be in pain or discomfort if it is. They are good at hiding illness because they're prey animals.
Hamster fur loss
If you do take him to a vet make sure it's an exotic vet that knows about hamsters. Some vets just automatically assume it's mites and give mite treatment (without testing). The mite treatment is very toxic to hamsters and should only be used if absolutely necessary. A good vet will take a skin scraping and look under the microscope to see if it's mites.
I wouldn't suspect mites unless he has been scratching excessively to the point of making his skin sore (doesn't look like mites to me).
Have a look at the link - it shows different patterns of hair loss - it may just be age.