Tricky one because the decision also depends on whether your son may need the mobility part of PIP for other expenses, for example he may make use of it to enable him to pay for activities, days out (which you may well provide yourselves) that type of thing or indeed to fill the gap left because he will lose the daily living payment. (its far from certain he will lose daily living but can be complicated).
I would say sit down and work out finances before committing the whole of the mobility payment to a car.
Hopefully there will be some more sound advice with what the best way to go is, and maybe some experience from people who have been in the same position.