I would suggest a vet check up before anything else. You need to make sure he doesn't have any diseases or disorders. Also talking to the vet about his "overweight" issue would be great, just make sure to do it somewhere Hammy can't hear, you don't want to wreck the poor little guys self esteem! :)
Anyways, you could try a different food. Maybe one formulated for overweight Hammy's. Also make him do exercise. Put him in a ball and let him run around.
The best thing to do would be to help your hamster shed some weight. You can do this by enhancing exercise. To support exercise, a large safe wheel that has a solid running surface is one of the best equipment to use. You should also avoid feeding your hamster with products that have much sugra. Visiting a vet is another great idea.
The first thing to do would be to take it to the vet to get evaluated. You should then adjust his diet. What are you currently feeding? A lot of commercial hamster foods are very high in sugar but low in proper nutrition. Look at this website to find a suitable food: http://hamsterhideout.com/forum/index.php?showtopic=71864 The next thing to do is consider its living space. How big is it? What toys are in there? I'd suggest making his/her house as big as possible... not small connected spaces, but one big open space. When I say as big as possible I mean you can get the biggest Sterilite plastic bin for $15 and modify it into a better cage than those $50 tiny cages at Petsmart. Then put a bunch of toys in there. Do you have a wheel? Make sure it's big enough, and NOT wire, because wire hurts their feet. So at least 8 inches big, preferably 10 or 12. Your hamster should not bend its back when running. (you can also look into Flying Saucers.) Lastly, get your hamster out and play! Make toys out of cardboard and Popsicle sticks.