Welcome to HC & congrats on your new ham.
Hamsters don't get dandruff & mites tend to be first apparent around the face & ears, this looks as though your ham may have been rubbing against something possibly, maybe a wheel or a tube that's too small.
Dry skin patches can also be a sign of some nutrient deficiency so making sure he has a good diet is important.
Do you know how much he weighs yet? He doesn't really look overweight as far as I can tell from that photo, hams rarely overeat & if they do put on too much weight it's more likely to be from lack of exercise so a good size wheel (at least 8") that runs smoothly & easily, plenty of enrichment in the cage to keep him busy & out of cage time when he's ready should solve that problem if he is a little too heavy.
Hamsters must always have adequate food available plus a hoard which he will make somewhere so don't be tempted to put him on a diet & cut back on his mix, do avoid too many treats though.
What are you feeding him? A good dwarf specific diet like Burgess dwarf hamster harvest or
Bunny Dwarf HamsterDream Expert is important, they are prone to diabetes so avoid anything with sugar or honey & only rarely if ever feed fruit, fresh veg are a good addition, starting every few days then building up to daily.
It's also important to provide a sand bath with chinchilla sand (tiny friends farm brand is best) that's big enough & deep enough for them to roll & dig in, that helps keep their skin & fur in good condition & also provides enrichment as they love it!