11 year old Grandson being violent & aggressive in the car

My 11 year old Grandson has started becoming very aggressive & violent when in the car. This has been going on for a few weeks & has gradually got worse. On Wednesday he went back to school, and he was so aggressive in the mini bus that the driver & escort said they wouldn't take him again. Once he got to school, he was fine. Charlie is non verbal, so this makes communication an issue.

If he is in the back seat of a car on his own, he will bang the windows & doors, and kick the seats, but over the past few weeks he has started to undo his seat belt, climb in the front & smack who ever is driving in the face, nip, bite & kick. Obviously this is dangerous to the driver, as well as anyone else in the car. A couple of weeks ago he did it to his mum, and knocked the car out of gear, she also hit the pavement.

Today, he was in the car & hit his 17month old sister in the face, and attacked his step mum constantly until they got home, as she was trying to keep him in his seat. He normally does not attack his siblings, but if they are in the car, he does.

We have tried to distract him by letting him play with a phone or have a personal DVD player, but that is not working either, and normally this would calm him.

He has become more anxious the more people are around, and this can cause violent outbursts in general, however they are far less violent than his behaviour in the car.

Any help, suggestions or advice from anyone that has had a similar problem would be greatly appreciated