Personally, i feel that if you go and change the brand now, it will just add fuel to the fire.
The children will notice the change of branding, and realise that they have had a negative effect on your son. And bullies feed of these reactions.
And like i said before, next week he'll be getting bullied for something else. Then when you change that, it'll be for something else. If he goes and changes his lunchbox to please these brats, they'll see him as an easy target.
I think you need to work with your son to understand that some children are just little shits and that he shouldn't want to be friends with them in the first place. Build his self confidence up a bit, get him to act like he doesn't care. I teach my daughter to just give a
expression and a derisive laugh when people say nasty things to her.
And yes, please speak to the teacher. Just as i imagine all parents would do if their child was being picked on.
I expect the fees alone are a fortune. Why on earth should you have to pay out any more just to please a bunch of nobodies? An aldi juice box won't affect your son's education; his low self esteem will. Focus on building this up instead of making his lunch more expensive. Otherwise next week, it'll be the wrong shoes. Then the wrong coat. Then the wrong haircut. It'll be never ending. Teach him the skills to deal with these snobs and talk to the teacher. It'll be far cheaper and far more effective.