Some men who are not very muscular, may develop the reverse of anorexia nervosa. They cannot be big enough and resort to unhealthy and potentially dangerous methods to build themselves up. They are likely to become addicted to exercise, pushing themselves beyond the limits of their body and often sustaining injuries.
Boys and girls are fairly similar in body shape until the onset of puberty, when they gain weight and body fat. This is critical for the production of reproductive hormones. Girls gain approximately 18 - 24 % body fat which tends to be stored around the hips and thighs.
Boys tend to gain about 14 - 18% body fat and it is stored more evenly in lean muscle mass, but some boys put pubertal fat on their hips and chest and this can evoke teasing, which is likely to erode their self esteem.
In general men will accept a higher weight than women if they are happy about their shape. Boys who develop early and become muscular are probably less likely to have problems than girls who start to develop the shape of a woman early, with a bust and wider hips.
Males with eating disorders struggle with many, if not all, of the issues that females do. They are affected by societal messages and expectations which are impossible for them to live up to. They want to act like "real men", but are confused as to what that means. They are required to be physically strong and yet tender, sensitive and caring.
Men respond equally well to treatments and approaches to recovery as women - if they feel able to admit that they have an eating disorder and seek help. Asking for help is difficult enough for women with eating disorders, but so much harder for a man as some people hold an illusion that eating disorders are female illnesses. It is also more likely that his condition and symptoms will not be taken seriously.