If your dog's heart is enlarged, he could have right-sided congestive heart failure. If, as a side-effect, his abdomen is accumulating fluid, then he won't be able to eat. What else did the Vet say? Did he place your dog on any meds? I'm curious to know if your dog is elderly?
My apologies, but I'm afraid it's not possible to offer any advice with the very few details you have provided here. Please give us as much information as you can so that we can attempt to help you.
Ghilly