Need advice please

Morning guys. 

So in feb last year I had my 3 grandchildren living with me as mum was struggling. Grandsons age 6 and 4 and granddaughter age 8.The 6 year old is waiting for an assessment for both ADHD and ASD.

2 weeks ago mum took back 8 year old and 4 year old but felt she couldn’t cope with 6 year old so he’s staying with me.


Here’s where I’m struggling as his behaviour has got more violent. His meltdowns are more frequent and he is physically hurting others at school, home and outside. Im sure it’s connected to all the change recently, new class, new teacher, new ta and the loss of his siblings. How do I navigate this ? He hurt a stranger and a family member over the weekend and while he is in meltdown he’s not capable of listening to me but when he has calmed down he either doesn’t remember what’s just happened or he won’t talk. How do I talk to him about consequences? Or right from wrong? when is the right time ?