My son has told me that the friends he has had in school since he started Y7 have all decided to group together and unfriend/bully him. This includes two boys who previously weren't friends, but have now joined forces to exclude my son from their social group. They undermine everything he says/does, and have even managed to turn his friends that go to another school against him- this is happening at school and also when he goes on his Xbox when he gets home. One of them seems to be determined to be the self elected 'head' of this group, and pretends to be nice to my son, whilst causing him trouble behind his back- and what also troubles me is that this kid smiles and waves at me at pick up time- knowing he's stirring trouble for my boy 
(he's even following me on Instagram)... And the kid he's teamed up with has a history of being a heavy handed bully.
I've advised my son to ignore it all and make new friends.... But I worry these kids will just keep causing him trouble. Any advice appreciated, I'm tempted to approach the parents- but I know the mums- and their children can do no wrong in their eyes 