There is a mug sat on my desk which says.
Do something, Do anything, but do not do nothing.
I´m not in child protection per say, but this is a huge giant red flag.
my thoughts.
This happened BEFORE you had a meeting to sort out moving forward with combined tactics - NO ONE TOLD YOU.
your ex tried to hide this from you
your kid behaviour is deteriorating
you have not had a chance to disguss this but you DC is back with the same people that did this and hid it? YOu will have NO direct contact with him untill the weekend?
For god sack. SOMEONE needs to be told.
I would start with SS, they can talk to you first, and then to your ex/np and the child
Oh and bullying at school? Maybe he ment bully at home, or maybe like virtually every kid I know he was worried about making everything worse.
Damodad, please at least run this past social services informally if you must and see what they say.
Hope that helps