It's actually not that hard for a gainer to down 5500 calories in one sitting once they get used to the miserable feeling they experience gorging like that.  A bucket of KFC chicken, a family size side of baked beans, mac and cheese and potato wedges and four biscuits comes to around 5500 calories.   Once he gets to 600goingon700 size, that will be breakfast.  That will be what he eats when he gets peckish after downing a couple of pizzas and a pan of brownies.  It's obscene and gluttonous and would kill most of us within a month to eat that way but 30k calories a day to gainers is not uncommon.