@Elliot Could the reason that the car hadn’t been ordered simply be the fact that the Motability system wouldn’t have accepted an order for an i4 until your son was 25, or does it allow an order as long as the delivery date is after the customers 25th birthday?
In fairness to Motability, I don’t see what they can do to help. If the dealer has made an error then it is down to them to resolve it. Hopefully they might be able to nab a cancelled build slot from another dealership as my BMW dealer did with our current X1. This resulted in us getting our car in 3 months when, at the time, others on this forum were waiting up to 7 months. Either way, it’s a shame for your son. Getting a new i4 at just 25 must be very exciting for him.