Need help with a bully?

i am 15 years old and a freshman in high school. i am short 5'1 , and skinny and have no muscle. the bully i have is also 15 is 5'6 and seriously fat like morbidly obese i am in his gym class and when we weighed him he was 300 pounds. everyday he pushes beats me up calls me names and one day he sat on me i thought i was gonna die. he always steals my lunch which leaves me very hungry. i do call him names back and run and he is sooo fat he cant even run, so he has his friends chase me down and hold me down so he can catch up and beat me up. he is really stupid, but he is going out with the hottest most popular girl in school (although she is a complete slut), and he has had sex with her and its true cause they both admited to it and bragged about it to everyone. so i cant make fun of him for that. i dont want you to say ask an adult because i am not a puss i want to handle this my self. what can i do