Can people please stop banging on about how he shouldn't have WhatsApp, it's a bit late for that now, and how will that help the OP? 🙄🙄🙄
OP, I went through very very similar with my daughter at the end of year 6. She was in a couple of WhatsApp groups and bullying started. Telling the school did nothing, as the class had already had the assembly about social media.
I was desperate, so I took screenshots and contacted a couple of the parents, showing them the proof about what was happening.
They didn't want to know. Despite the evidence their attitude was 'not my Freddie, he would never have done such a thing' etc etc, and my daughter was blamed (no idea how as she had not done anything). And I was friends with these parents, but they quickly turned.
I don't know what the answer is OP. My daughter was desperately unhappy until she started secondary school this September just gone. She's now got new friends and is so so happy.
I'm sorry your DS is going through this, some children can be so horrible x