Smita, I am not a mom but I have been teaching 6 year olds for 8 years. I would take this seriously and talk to a child psychologist or therapist. If he is threatening to harm himself there is definitely an issue that needs to be dealt with. He may need therapy or may have a chemical imbalance that medication could treat. I wish the best of luck and give you credit for posting your question!
Could he be upset by anything has he any siblings, What happens prior to him becoming aggressive , and stubborn, what reason would you think he would act out that way. Children often repeat things thats okay , he says he has to go to school or he will get more home work, how does he get on at school and how much homework does he get at 5 year old. Fussy eating is okay , let him choose his food when you are at the stores , they like to do that .