DS1 is 4, started reception last September and is one of the youngest in his year group. He loved nursery, was quiet but seemed to get on well.
Reception started well, loved it at first but over the months has been more reluctant to go. He wouldn't really say why, until I heard from his teacher that he was pretty quiet. I asked him how he was doing, who his favourite friends were etc (I ask him this a lot and he's sometimes reluctant to answer or goes off on a tangent). He finally told me that he doesn't have any friends, nobody likes him
and if he tries to play they tell him to go away. I told him that he could go and find some better people to play with but he told me no-one in the class will play with him. Some of the boys push him and call him stupid.
One day a boy pushed him into a railing and he hit his head, my son found and told a teacher but told me "I tried to tell her but she ignored me and walked away."
Now, one of the problems is that when he's talking to the teachers he is apparently very quiet and won't make eye contact, and is a bit difficult to understand. I can see how the teacher might not have noticed him, but feel so sorry for him. he came home with a huge bump, and the teachers had no idea 
What can I do about all of this? I feel so so bad for him, he is such a lovely, sweet, kind boy, bit too dinosaur mad at times but very caring and funny. At home he's a ball of laughter and energy, chats non stop at ear-splitting volume, but he's so different at school. I've been telling him that if other kids call him stupid he just needs to tell them in a big voice that only stupid people say stupid, to tell a teacher if they're mean to him or hurt him, and to speak clearly to the teachers, and that most of all he is brilliant and lovely and funny and deserves the best friends in the world.
I'm meeting his teacher soon to check they're keeping an eye on him especially with the pushing.. I know so much of ths is normal playground stuff, and 4 yr olds are not the most reliable source but he seems so unhappy. Anything I can do to make it better for him?