"old enough and smart enough"
Really?
You've never met him, you don't have a clue what he's like. Lots of countries (Scotland included) treat under 25s differently in terms of sentencing guidelines etc because it's recognised that brains aren't fully developed until then. I don't necessarily agree with that personally, but it's a fact that, no matter how smart they are, most 16 year olds have very minimal life experience.
I don't think he could really grasp the concept of how not going to school might impact him long term, not just in terms of his education but long term impact on mental health, friendships, etc.
It's easy to think 'I don't need it I can just do my a levels later' without realising how many college courses have been cut, or how he might feel being 5 years older than all the other students, or being bright but being taught in a group of people for whom English is their third or fourth language, or who have failed their exams multiple times already because they struggle with learning, and having to go at their pace.
Or 'I don't need qualifications to be a professional x box live streamer or influencer' without realising how few people make enough money from those platforms to live on.
Or 'I'll still have my mates anyway,' without realising how quickly people grow apart when his friends go off to uni or go travelling on the money they are earning from working, when nobody will hire him because he doesn't even have GCSEs.
He's never paid bills. He's never lived away from his family. He's never had a job. How on earth can he "know" the way his choices might impact him ten or twenty years down the line, most adults don't!