Your boyfriend sounds like a jerk. Tell him to shut up.

I also have a lot of issues putting on weight. I'm 5"7 and 111lbs, but I eat as much as a full grown man. I also get sick of snide remarks about eating disorders.

However, according to my physician I am on the (very) low end of what he feels is a healthy weight. I am comfortable with my body and as long as I am healthy I refuse to care what anyone else thinks. I'm afraid I cannot give you any advise about gaining weight (as I haven't had an success myself), but I advise you to see a doctor about your weight. If (s)he is unconcerned I wouldn't fret.

Edit to add: having read all of the posts it seems as though you are concerned about sudden weight loss? Have you had a blood panel done? When I was anemic I became even more thin, but weight loss was my only symptom.