My son is in Kindergarden and he is getting picked on by another kid in his class what should i do?

my 6 year old enrolled to school last year. he is in grade K. I have met his teacher and she seems really upfront and nice. My son has came home last year from school crying saying that a little kid in his class had pushed him when he was at the top of the slide and he went sliding down but had hit his head on the side of the slide and got a scrape on his elbow. This was at recess time when it had happen. He told me when I went to go pick him up what happen and so I went to talk to his teacher and she said he didn't tell anyone. he should of told someone about what happen. I told her will I am just telling you now of what had happen and I don't like it. so she says she will talk to the other child but my son ever since then has been scared to go to school, always saying he has something wrong with him so he can go to the nurses office and hope that the nurse will call home and have him get picked up. He is potty trained but early on in the school year he would urine in his pants just to get out of class and either come home or have me be there with him for a bit. he don't do that no more but the kid in his class has still been bothering him. I went over with my son and his teacher a lot of times of what to do if someone is bothering you and he has it down just he is shy so he don't tell. He sometimes won't tell me if he was bothered at school that day until a week or so later. everytime he has told me though i had went to talk to his teacher. she said that other kid is seeing a shrink and they are trying to work with him at the school too but my son is still scared to go to school. I don't want my son being scared to go learn especially when he is just starting. should i address it to the head boss there at the school or what should I do? My son went from crying every morning to waking up asking if he can get a nerf gun so he can shoot that kid and let that kid cry. i don't want my son to be thinking that way. I have no idea where he gets it from I don't allow tv only disney movies for 1 hour or so then its doing activities. My son says he has no friends at school and his nephews who comes over don't want to acknowledge him unless he has something cool that they want to play with. When he tries to play with them they would either push him around or just run and ignore him. It hurts to see my son feeling so sad and crying in the bathroom sometimes when he is getting ready to for school or for bed. I talk to him everyday but i don't know what to do. he is only 6 years old. can anyone help?