From personal experience it is very hard to eat let's say 3500 calories if all you did was eat 2000 calories before (a day). You have to increase frequency of your meals and slowly increase the actual size of the meals. Soon enough your body will be begging you for food every 2 hours without you needing to force feed. Also use protein shakes mixed with milk or juice, they spike the sugar levels and make you even hungrier later on.