Definitely 100% sure it will be a bad fit. He is quite small and the next 2 classes up terrify him due to several older and large boys being held back due to intellectual disabilities ( think 20 Kg heavier ) 8 years old in a class of 6 year olds for violent behaviour.
Do you mean the private school has violent kids 20 kg heavier than him?
If so, then I see multiple alarm bells screaming DON'T MOVE THAT KID. If I'm wrong, ignore the following, but I would advise in the strongest possible terms against moving a child out of a happy situation to a dangerous environment just to make a sibling happy. Find another solution. Or the sibling can just wait and you can figure something out in the meantime.
If the current school is the one with the violent kids, then yes, move him.
