Hamster hair loss is usually a vicious cycle: some irritation causes itching, itching causes scratching, scratching causes more irritation, etc. Other usual suspects would be stress, which can lead to both hair loss and scratching, low food variety, allergy and cancer.
While I wouldn't be alarmed, it is always a good idea to take him to a vet.
I personally would rule out cancer. While cancer appears too frequently in hamsters due to inbreeding, 7 months is a little too early.
Irritation is usually more localised. Could you add a photo of the bald spot? If it has well defined edges, I would say irritation would be more likely. There are some over the counter creams or lotions in petshops specific to hamsters that would help you with it. If it is like male pattern baldness, i.e., there is a spot with less hair and the hair gradually increases to the edges, stress, food quality and allergy is more likely.
Packaged hamster food is usually food scraps pressed in shapes and sizes with some nuts and dried maze added. While, they are safe to use, never rely solely on them. You can add shelled hazelnuts, which they can open themselves and it is a lovely sight to watch them open one, cracked walnuts, cashew, whole grain bread in moderation, very small pieces of boiled egg yolk, some leafy vegetables like lettuce, carrots and bones. Make sure that the vegetables are at room temperature and are not wet and bones are washed up to remove any grease. Grease and wet veggies may lead to diarrhoea. If my hamsters' coat loses their quality, I would increase the boiled egg yolk.
Stressed hamsters usually behave erratic, i.e., more erratic than a regular hamster. Randomly jumping to the air, not leaving the nest other than drinking and relieving himself might be signs of the stress. Make sure that you have a properly sized container for your friend. The base should be at least 0.13 m2, so some 30x45 cm cage with a height of 30 cm should be enough if you only have a single dwarf hamster. Dwarf hamsters also do not like to live in solidarity but you can't buy a new dwarf hamster and add him into the cage, they need to be together since birth, so there is nothing you can do about this. The way you handle might also cause some stress. When you put your hand in the cage, let him climb on your hand rather than you picking him up. Never ever ever open up the nest. Nest is a very private region for your hamster, and this induces extreme amount of stress. Don't even clean the nest, just check for spoiled vegetables and remove them.
Then again, consulting with a vet would be safer if this continues.