The first thing I'd do is google that Sunday Times (?) article on racism, school, and Lewes that appeared a while back. use those key words. Warning: It may be not have ben in the Sunday times, perhaps Guardian???
You need to be clear about where you stand on this, because it is about how the school deal with race issues, and that may well mean you are going to have to be the educator here, and will have to be, sad to say, quite clear yourself about how defensive the school might be.
At the risk of lots of people jumping on me, I do not think this is a teacher issue, I think it's a Head issue.
If it were me, I'd be, gently at first, making the very clear point that this is a longer-embedded issue, and has its roots in the question about why your son didn't feel empowered to raise the issue when it first occurred. I'd point out that that has led to its being subterranean for an entire year.
A school should not be a place where a child feels alienated enough not to be able to raise issues of racial abuse/racism, and for that to seep into their home-life, and to internalise, at home, that they must keep secrets about the depth of problems at school.
I'd be saying that I realise this is an issue we all must work on, for the good of all.
I'd say that the teacher's inability to deal with the "tail" of this speaks loudly of a structural problem, which in and of itself means that it cannot be corralled, and cauterised into a limited, series of incidents: one occurring last year, and "done with", and one occurring this year, and being about disobediance/rudeness.
I'd make it clear that you are happy to deal with the infractions of rules etc. that your son has displayed, but you are going to need some serious help from the school in dealing with the wider issues, because otherwise you know, and they know, you are going to be sitting in this office again.
I'd be prepared to go through the negative narrative, too, of how being the target of obvious, or even oblivious racism, impacts on children's behaviour, and you don not want your son taking that route.
Good luck. At least he's told you.