I have firsthand experience of bullying, and of being the parent of a bullied child, and I know how much it hurts.
Firstly, you have handled this the right way - talking to the school and letting them deal with it. It may be that you haven't been told about the shoving incident because the school had dealt with it and schools often prefer to keep things 'in house' - they don't want children going home and getting in trouble again at home if they've been told off in school, is what I was told. They were probably monitoring the situation to make sure that it didn't recur.
With regard to the racial nickname, again, I suspect that they want to talk to the child concerned and monitor the situation. It may be that the child concerned is just repeating a word that they've heard somewhere else and it doesn't necessarily reflect a racist attitude in this child, iyswim.
If the incident is a 'one-off' and is not repeated after they've talked to the child/ren concerned, then the school has successfully dealt with the situation, and hopefully you and your ds can draw a line under it too - though I am not in any way attempting to belittle the hurt you are both feeling.
It might be worth asking the teacher if this subject could be addressed in 'Circle Time' - so the whole class can be made aware that using nicknames based on race, gender, hair colour etc etc is unkind and unacceptable.
Just one thought, though. A single incident of pushing or name calling isn't bullying - it's nastiness and needs nipping in the bud, but imo it is when the behaviour is repeated more than a couple of times that it turns into bullying - which is why I say it should be nipped in the bud at the first or second instance.
Children of any age can be nasty, in my experience, but not all of them are bullies by any means.
To illustrate this point - my ds2, aged 13, suffered a horrible incident where his pe shorts and underwear were pulled down by another boy in pe. He was absolutely mortified and upset as you can well imagine, but the school dealt with the incident so quickly and firmly that he felt reassured that he was safe there. It also helped that his friends rallied round and supported him straight away, which tells me that the school has a good supportive ethos and that bullying is not tolerated by the children either.
In the wake of this incident, the boy concerned had to come to ds2 and apologise, and since then they have been on reasonably friendly terms.
I hope that you and your son have a positive outcome from this nasty situation, and that things don't escalate into bullying for him. He is very lucky to have a mummy who has reacted so quickly and positively to support him. Although I was older at the time, when I went to my parents in tears at the sustained campaign of bullying in my final year at primary school and then in senior school too, they told me that sticks and stones would hurt my bones, but calling names couldn't hurt me!! Utter nonsense, and after that I didn't even think of going to the school - after all, if my own parents weren't going to help and support me, how could I expect the teachers to do any better?