Aside from evident weight loss, red flags that may be indicative of eating disorders in men are peculiar eating habits, overeating or binge eating, vomiting their food, and skipping meals, especially if the men in question have never exhibited these behaviors before. Men who have eating disorders may also be depressed, have low self-esteem, have a misguided sense of what is healthy, and feel pressured or confused about how other men (should) look, among other things.
There are sufferers who may be overlooked or considered normal but may actually be already suffering from eating disorders. These include men who, although they exercise a lot, do not actually eat. Sadly, the major stumbling block in determining if a male loved one has an eating disorder is their tendency to become secretive, their refusal to talk about their problems, and their need to conform to societal norms.
If you suspect that a family member or someone close is suffering from an eating disorder, you better talk to that person straightforwardly and in a non-threatening manner, with the goal of encouraging him to seek help.
But before even attacking the psychological or psychiatric cause of the problem, it is probably wise to bring the person to a primary care physician, so the doctor can address the nutritional aspects of the disorder first and conduct further tests (such as those for endocrine and digestive functions) if the clinician suspects that the case is not related to an eating disorder. The goal, at the end of the day, is to bring back the male loved one to an optimal nutritional state, ideal weight, and a healthy and sound psychological and emotional state.