Don't force him to clean up his plate. Desserts should be as healthy as the rest of the meal so should not be used as rewards. It has been proven that if one food is used as a bribe to get a child to eat another food it results in the child disliking the food he is bribed to eat and liking even more the food that is being used as a bribe. If a dessert is so rich that it is not as healthy as the rest of the meal, wait until the kids are asleep before you eat it. Don't have unhealthy food in the house.
Kids can always find it.
Food is not to be used as a reward, a token of affection, or punishment. Never allow fruit juice to be used as a between meal filler. Use them only at scheduled snack times. The current recommendation about fruit juice is that it is not t o be offered to children under the age of 2.