Is not following the father's orders to reduce bread and sugar considered disobedience, even though the father does not get angry about it, but his son fears that following his orders might lead to thinness, which could cause his father to order him to stop voluntary fasting, taking into account that the father advises this to preserve his son's health?

If the father advises but does not command with obligation, then there is no need to ask about the necessity of obedience and the sin of disobedience. Obedience to the father is obligatory if he insists that his son break a supererogatory fast. However, obedience is not obligatory if he forbids his son from eating permissible foods and compels him to eat specific foods, though it is better to obey the father and respond to his command.