What can i do about this boy that i like and he likes me to but hes my best friends ex?

me and my best friend have been really close till just now i really need help cuz one day hes giving me hugs and telling me that he really likes me but then he goes to theria witch is hiz x and mii best friend tell her that im fat but i dont know who to belive cuz when i ask him he says he dont know what im tawking bout but when i tell theria she says that hes lying and i need to let him go cuz he aint worth it and i waz stupid for likeing him when he calls me fat all the time and im not fat but im phat or thick plz plz plz plz help ill give the best answer alot of points