Very beautiful dwarf
. I also have a dwarf that's over weight, and has seen vets. They can gain weight, and gain the weight pretty quickly. If you can weight your dwarf, to see how much he weighs, that's a good start. Read the ingredients of the hamster food- no molasses or honey, is good for dwarfs, as the others have said, dwarfs are prone to diabetes. Also, if you are feeding him a mixed seed type of hamster food, he maybe selectively picking out the seeds that are higher in fat, and not eating the rest, OR the portion you are giving maybe too much.
With my overweight dwarf, I'm feeding 1 tablespoon of hamster food, and although she does not like to exercise, taking her out of her cage a few times a day, to let her run around and play in a pen with hamster toys. My dwarf is on a protein, fiber and low fat diet, in hopes of helping her lose weight.