I agree with everyone who has said you should see the teacher - pleased to see you're going to.
But you also need to give him some help in dealing with this sort of thing himself. Bullying takes hold gradually sometimes and he needs to learn to nip it in the bud. I knew people at school who were never bullied and others who were endlessly being picked on and there was nothing between them except the first just didn't bow to anyone or shrugged it off somehow.
Whether it's learning a stock retort, thinking how to handle it or learning to 'get the laugh in first' even, talking through a strategy may help him.
My DS (year4) has a heart condition which means he is always the slowest at running. He always will be. Nothing is going to change that. And we know he will be the last kid to be picked for team games (who wants the slow runner on their team?)
He was very upset after his first sports day as he was only halfway down the track when the others had finished. The parents at the school all know him so they were cheering him on - not one bad taunt from another child in reception at the time. However, we spoke to the consultant at his next heart check up who said 'You just being on the track is a greater achievement than the kid who wins the medal easily - remember that'. After that DS adopted 'Waterloo by Abba as his anthem - because it has the line "I feel like I win when I lose" and we sing it before every sports day to psych him up. Last year he came second to last in the obstacle course! Atta boy! He was so proud and so were we.
We also make sure we point out that it's his brain power which is going to help him in future - not the ability to run fast. Not many people get a job involving fast running - most people need a fast brain rather than fast feet in their work. He came top in his year for his spelling test (100% correct) and got an award from the head. That's helped him secure his place in the class eyes. He now gets asked how to spell words and people want him on their table because of his ability - so he's not the athlete but he's the kid who knows ' stuff' and is willing to share that knowledge and that's made him popular.
A few years ago one kid taunted him about being slow during the sports day trials. DS turned to him and in a loud voice said 'I have a heart condition, I'm a slow runner. Get over it - I have to'. DS's friends cheered, bully went away sheepish and taunts stopped.
We try out different retorts or roleplays with DS if there's a hint of 'nastiness' going on, even if it doesn't involve him. So we'll say something like "so if that happened to you what would you do?" Sometimes it's go and find a grown up, sometimes it's ignore it, sometimes it's laugh along (that can be powerful too - I learned that at school, getting in first with the laugh defuses the bully sometimes. Seeing you upset is what gives them power - gives them a kick. Seeing you smile and say 'yeah, you're right, oh well' doesn't give them a way in and they'll pick on someone who gives them the satisfaction they crave) sometimes just a quick 'that's really mean did you realise that?" can be enough at that age.
It's just giving DS an arsenal of ammunition to a) send the bully packing at an early stage and b) give him the self esteem to shrug it off and forget it.
I hope you get things sorted out. I'm from a family of gingers and had a ginger friend at school who was the most popular girl. And think of all the 'would be' brides lining up for Prince Harry! Ginger makes you stand out in a room - get the personality behind it and you can blow away the opposition!