Just a bundle of random thoughts:
Start with glider license and venture into powered gliders aka “Touring Motor Gliders”.
This will lead to a Private Pilot license that may lead to an opportunity to become an (unpaid) glider tow pilot.
Flying vacations to the likes of Poland, Hungary, Czech Republic, South Africa, Canada or the USA for cheaper time building.
As a side note, the FAA (USA) counts glider time for 100% towards a Commercial Pilot certificate and the ATP as the FAA considers any airborne time.
https://www.law.cornell.edu/cfr/text/14/61.129
With careful planning during the time building stages most requirements can be met relatively cheaply.
After gaining the PPL you need to consider the flying required to maintain your skills at a reasonable level. For the sake of the discussion let’s say 50hrs/year.
That’s between £6-£7k/year.
He’ll need a job on the side that pays a significant portion of this amount.
How much you are willing or able to help financially is between you and your son.
Be very careful when it is appropriate to take out a (small ) loan.