Horrible for you, but kids are cruel. I would say, do nothing right now as far as speaking to teachers is concerned, it could be a one off and as your son has told you long after the fact, its in the past.
Look for repeated instances though, ask your son every day how his day has been and has he had a nice time with his friends, don't ask him if he's been picked on though, let him offer you that information. Talk to him about self confidence and NOT believing/caring what others say.
My son was in this situation last year, he has inverted nipples and whilst changing for PE got mocked for having 'boobs like a girly'. Obviously it upset him, and we had a chat about how to deal with it. Left it at that, it didn't happen again.
However, later in the year, a boy in his circle of friends started calling him fatty and stupid... this happened regularly over a short period of time, I spoke to the class teacher, I didn't use the word 'bully' - I firmly believed this was a case of my son and this other boy having very little tolerance of each other and my son reacting to the first instance badly, then it became habit (my son tends to flounce, and it IS pure comedy gold to watch) because the other boy has got the reaction he wanted. NOT NICE, however, not bullying. It could have turned into that though. This other lad is actually a nice boy.
The class teacher had a word with both of them, got them to explain what happened, got them to shake hands and start again. Its never happened since.
You've got to be on the ball. Sometimes one name call and reaction will be enough to start the slippery slope towards bullying, sometimes its a one off. Its very important that your son TELLS you about it as soon as he possibly can, to give you a realistic gauge of what's really going on.