Male hamsters can have issues with fur thinning and loss as they grow older, similar to male pattern baldness in humans. Is he scratching a lot? Do you notice small reddish bugs on the skin? This can be an indication of mites. What kind of bedding and food do you have him on? Some hamsters develop allergic reactions to a bedding over time, though this may be accompanied by other symptoms. Too much protein in an older hamster's diet can also lead to fur loss so this is something to keep in mind! Pictures would help too
