I feel for you & your DS :(- my own 9 yr old DD is going through similar right now - though in her case the Girls WERE all her good friends, but on getting back to School late this term due to being very ill, under the lead of a similar character to what you describe, they've all turned on her & like you describe, its very subtle, but incessant picking away at her - she also has IBS, so its affecting her health badly - again the School are being good & are trying to resolve things in a low key way, though I do sometimes wonder if they might be tougher if one of the key players wasn't a staff members DC, but they are trying various strategies.
One thing which I am finding helps my own DD be more positive about the situation is the phrase
"Bullies, Bully You, not because the hate you, but because, they are very insecure & either want to be you, or you remind them of something they don't like about themselves - so its actually all about THEM & the way the see themselves, not YOU, so don't let them win & let it affect you -
I also find times like this morning, when my School loving DD was crying hysterically that she didn't want to go to school & face them, talking to her about dressing smart, in a way she feels good about herself, helping with hair, clothes etc, being like putting on a suit of armour - it won't stop them, but it makes her feel stronger in herself - IYSWIM - after checking herself in the mirror - she left with a smile on her face - is this something you can try with your DS ??