Your son has two things:
The privilege of time.
The privilege of changing his mind.
He’s 18, the chances of him changing his mind as far as a career path are at the very least 50/50. He may or he may not.
If he does you don’t want to be looking at paying off a £50k-£80k debt…or more.
Consider the privilege of time and pick an almost arbitrary year, 25.
That means he has 7 years to achieve his current goals which makes him a perfect candidate for modular training.
Current PPL, some time building in the US or Canada or South Africa next year.
Instrument rating somewhere 3 years from now, take 2 years for the ATPL exams.
Send him off to college or university and just keep up the flying in between with priorities given to advanced education.
He’ll have a degree and be ready to start applying for flying jobs anywhere in the world at 25 and with some planning mostly debt free.
Nothing is for free in life so he should get part time jobs to at least pay for a portion of it himself. How much is up to the two of you to decide.
A part time job that pays $€£500/month means 6000/year means every two years he can do the next license/certificate or rating with some financial help from you.
I can elaborate more if you wish but that would be my recommendation.