A lot depends on the nature of the "garage" and what was said at the time of sale and any paperwork - was warranty optional or paid for etc.

A driveway with 2-4 cars for sale may have a business bank account. Reality may be a sole trader dealing in a few cars. If sold as seen - possibly no formal warranty provided. Even if there is a legal basis, actually recovering the money paid may prove challenging.

If purchased from a more formally constituted business there may well be some legal recourse - although even larger garages may sell trade ins on a sold as seen basis.

It is of no great comfort, but buying a 14 year old premium motor with 190k on the clock is very much a case of buyer beware. If your son was unable to sensibly assess the vehicle he should have taken someone who could.