This is ridiculous! If they have charged him with taking without consent how on earth can they also say you allowed him to take the car. These two things are mutually contradictory.
I suggest you get him to confirm to Motability in writing what has happened and send them a copy of the charge document he has from the police.
Also, you should use the police complaints machinery (I think it starts with a letter to the Chief Constable) to try to get them to see the error of their ways and confirm the true situation to Motability themselves. (Or you could ask to speak to the local Superintendent or Inspector for your area to try and get it resolved more quickly - from what you have said, it seems only to need someone to engage brain to get it sorted.) You might also threaten to use the disability discrimination legislation - the police presumably know you have the car because you have serious mobility problems.