You need to remember that dogs are animals, they cant express their opinions the way we can by shouting at each other they react in different ways, I wouldn't leave my dogs in the company of kids on their own even though I believe my dogs wouldnt hurt a fly they are still animals and they can snap at any time, not necessary out of rage but out of fright or excitement. As others have said you did do the wrong thing by telling him off after and sending him to the garage but I wont repeat what others have said. Just remember you cant punish an animal for something they have done a while back, I hope your brother isnt too shook up and I doubt that Rupert meant any harm it sounds like he was more frightened than anything.

EDIT..

I just read about your mum having him put down? what happens if something else scares him? if your mum has him put down she doesnt deserve to own a dog or any other animal.