Safety harness

I am raising money for my son Macaulay. He is registered blind and suffers from autism. He enjoys going out for drives , walks and spending time with his family. Macaulay is currently unable to go out as his behaviour in vehicles is causing danger to himself and others.  He likes to get out of his seatbelt and jump in the front to try and drive the vehicle himself ! We have now been told that unless a specialist safety harness is fitted to vehicles that he travels in he will no longer be able to access activities away from home. He has been restricted to staying in for the past 3 weeks until a harness has been purchased.
We have had a demonstration from a specialist  company for a magnetic safety vest which Macaulay will find easy to  put on and will keep him safe and secure in the car.  We have been quoted £600 to have this fitted.