My Dog Has A Fat Stomach,what Should I Do?

If your dog is overweight you need to work with your veterinarian to determine the dog's ideal weight, body condition, diet, and exercise plan.  
If your dogs abdomen is suddenly very distended (not from overeating) then you need to have your dog examined by a veterinarian immediately.  There is a disease called GDV (Gastric Dilatation and Volvulus) or bloat that causes abdominal distention.  Without treatment bloat is fatal.  
Contact a veterinarian to determine the appropriate course of action for your dog.

If he is a big eater that could be the reason, but odds are you are talking about being bigger than normal. There could be several reasons for this. He could have an obstructed bowel, or constipation, among other things like his liver, or kidneys, you really need to take him to the vet as soon as you can. Hope this helps, good luck to you.

Then he is eating a lot. What is your question?

Since you don't say if this is a pup or a full grown dog I don't know what to say...