Under the motability car scheme, only drivers nominated on the policy can drive the car. There are no restrictions for the customer to drive or learn to drive in the car, but for you, the son, things are different.
You need to be 21 or over if you only have a provisional licence and want to learn driving in the mobility car and you're not the direct customer of the scheme. This assumes all other criteria are being satisfied by the way, that is, you will be supervised when driving and you meet the requirements of being a nominated driver (live within 5 miles of disabled customer address, no endorsements on licence or disqualifications etc).
Source : Motability Car Scheme website & email confirmation from them!