Oh, he is me, 60 years ago! I was that grammar school boy who knew better than his parents and teachers. Did bare minimum academically, scraped through O levels, played truant instead of studying for A levels, got kicked out, took dead end job on lowest rung of civil service, got transferred down to London, lived a dissolute life.
In my twenties, met the right girl, bought a house, took Civil Service exams, moved up the career ladder, finally retired as a Senior Civil Servant. Some of my grammar school peers, who conformed and got degrees, did less well in life.
So chill, let the boy make his own mistakes, show him unconditional love. He'll be all right ..... eventually!