I think a lot of you are being too harsh on OP's boyfriend, presumably because you've never been in his shoes.

Growing up, I was chubby and socially awkward while my little brother was athletic. He started dating girls in 7th grade, while I basically had never even hugged a girl until college. People constantly were amazed at how much better looking he was than me and how he was already on his 4th girlfriend by 10th grade.

Thankfully I was just suffering from ugly duckling syndrome -- after losing fat, gaining muscle, and growing a few inches, I'm pretty attractive nowadays too. But years of growing up with a sibling who easily overshadows you is ROUGH on the self-esteem. While OP's boyfriend is definitely taking it too far, I can definitely see where he's coming from.

Honestly, the only thing that can be done at this point is for him to lose the weight. It's a fact of life -- overweight people are less attractive than athletic/lean people. 100 lbs overweight is a LOT of extra weight, and it's pretty fucking obvious that an obese guy is going to be far less attractive than a muscular stud.