My best friend who is also my boyfriends best friend always is calling me fat... I struggle with an eating disorder and I have tried everything to lose weight... does he rele think I'm fat or is he just being a guy?

Ask him. Does he know that you have (or had) an eating disorder?

In any case, calling someone fat is rude. Unless you have specifically asked him "Am I fat, do I need to lose weight?" he has no business calling you fat even if you wobble when you walk.

Bear in mind that what a bodybuilder considers fat is normal to most people. I consider myself a non-competitive bodybuilder, and by bodybuilder standards, I feel like a fat heap. By School Mom standards, I'm in great shape!

My boyfriend knows that I have had eating disorders but my best friend doesn't

Would it be worth telling him?

Umm I'm not sure but I can try it do u think that would be a good idea?

He is probably just joking around, thinking he is being funny and teasing you, but if he is your best friend, I would hope he would be more considerate. I would tell him and let him know how it makes you feel. How he responds, and whether he stops the comments or not, will let you know if you should keep him as a best friend.
Since your boyfriend knows you have struggled with an eating disorder, I hope he would say something if he ever hears the comments.