What should i do please help?

My son is nearly 4 years old and just started nursery in september. He really enjoys it but now for the last week he has been coming home upset and saying a boy keeps hitting, biting and pushing him of bikes etc. It started last week when the teacher got me and said that another boy had bitten him and they had to tell me because it left a big imprint and made his hand bleed. I was willing to let it go as that what kids do and he had been told its wrong. The next day my son came home with a cut ear and he told me the same boy pulled him by the ear. My son is very quiet and didnt tell the teachers but someone else did but they did not tell me. I had to find out off my son. Yesterday he came home with a cut chin where the same boy scratched him apparently he told teacher but they stuill did not inform me. Are they taking this seriously as everyday he comes home with a different mark on his tiny body. This boy is alot bigger than him and he is now scared to go to school. I have informed the teachers but it does not seem to be stopping. What should my next step be, should i approach the mother or should i take him out? My son would never hurt anyone and i would not condone him hitting back what should i do?