Do not fret about it too much. Advice I would give:
Get a careers appointment if he can (his school should offer them). Ideally in Y10.
Try and work out what he might want to do. It doesn't matter if career wise he is unsure ..but maybe to understand if he want to do engineering he needs to focus on science/maths. If Humanities is more his thing, that is fine. If he can analyse and write a great essay, but is crap with a protractor...go with it...don't try and get a square peg in a round hole. Love for a subject is better than "I must do this even though it bores me"
Explore his options college/Sixth Form wise. Consider courses, friendships, commute, the vibe, Sixth form quizzes etc etc He will spend 2 years, hopefully of one of his happiest school years there. Don't think just academics. Would all his classmates live in close..or an hour then otherside of a college already an hour away. Can he get there on his own?
Open days. What is on paper is different to how they "feel"...though remember they will be in a bit of "sales" mode.
When he has his first choice, make sure he has a back up/third in case he doesn't get grades/they withdraw his favourite course/they are oversubscribed
Research their processes. When are interviews? When are inductions? When are enrollment days?
If he can get to more than one induction at colleges do. They can go over several days..and can clash (first week after exams finish for a lot). If necessary sign up to both....(they may say he HAS to be there)...and then just work out one where he is suddenly unavailable on day 2. See how it plays out.
Do not go on holiday on results day/day or two after. Really. Don't. You may not need to go into any school to enrol. But you might. Or you may need to go into to change a subject/discuss a waiting list/negotiate re the near miss of a grade. Being close to home rather than away makes this way less stressful.
Remind him it is exciting. And that ultimately his Level adoptions are down to him. That his a level results are how hard he works, not where he goes. But that also you love him. That he will be great, whatever happens, wherever he ends up...it is his journey, which may not be the same as his mates/what he thought etc