Based on the information provided, Max is exhibiting behaviors that align with Anorexia Nervosa. His severe restriction of calorie intake to only 300 calories a day, excessive exercise, and a body mass index that is below the average for his gender, age, and height are characteristic signs of this eating disorder.
While you mentioned that Max is male and 45 years old, it is important to note that eating disorders can affect individuals of any gender and age. Therefore, the idea that he is "too old" for an eating disorder does not hold true.
Bulimia Nervosa typically involves episodes of binge eating followed by compensatory behaviors, such as vomiting or excessive exercise, but the details you've provided do not indicate binge eating episodes.
Therefore, the correct answer is: Anorexia.