Work out in water. Or just watch what you eat. Honestly you don't even have to exercise at your weight. Just eat something resembling a normal number of calories.  Check a BMR calculator, but a quick and dirty check based on this post showed about 3000 calories a day as a BMR for a 500 pounds man of average height. You could lose at least a pound a week sitting on your ass and eating 2500 calories a day.