I would agree with Motability viewpoint. The son is not living at home. Whilst you can argue that having the car so the parents can take him out or visit him / take him supplies is for his benefit, there is a greater argument that it is for the benefit of the parents as it makes it easier and cheaper for them. As understandable as this is, it’s arguably not what Motability is for.
We come under a lot of unfair attacks on social media and people could argue this is no different to anybody who has a granny in a nursing home and has to use their own resources to visit them.