You have had some great advice, here is my take:
Remember that bullies are often covering up something else, perhaps problems at home, perhaps feelings of insecurity. The underlying reasons why they may be acting this is not your problem, but understanding that there may be other things at play may help you to ‘pitty' them and help you to take the moral high ground and rise above it, feeling strong within yourself.
This is a wonderful life lesson which you can gain incredible skills in dealing with such behavior. In the adult world it is just a prevalent, and the way to deal with it is essentially the same.
You need to send a clear and unequivocal message that you will not tolerate his behavior towards you without making the situation worse.
What might make it worse?
1. Message is too weak and taken as invitation for further bulling behavior
2. Message is too strong and/or causes embarrassment which angers him into a stronger vendetta against you
Here are a couple of ideas of the types of things you could say:
"I will not tolerate your behavior toward me, I will speak to Sensei if I have to"
"I find the way your talk to me incredibly annoying, do not ask me to train with you unless we can concentrate on working TOGETHER to get better at the techniques"
"You have no right to talk to me this way, I come here to learn and dont want you distracting me"
Keep it short, simple, non personal, statments of how it affects your training, not how it affects your feelings. Show no weakness, keep it clear direct and on point without getting personal.
Remember it is not just what you say that is important, it is the also ‘energy' (ki) and ‘intent' with which you say it. The energy that is transmitted within the message really is the key. As with Aikido techniques, the outerform needs to be just right but the inner, latent (unseen) qualities are really what makes it work.
You should look him right in the eye (metsuke), and say it from your belly (hara) in a strong, clear, direct and calm voice, with no hint of hessitation or fear, and preferable with a firm belief that he will listen to what you say and take it seriously.
Practice in the mirror or with your mum or dad.
If this doesn't work go to Plan B, tell him during/with a nikkyo.