DS1 is my first child, you may be able to tell, as this in new territory for me. I have professional qualifications, equivalent to degree, but haven't been to Uni so the process is unfamiliar.
DS is hoping (expecting, really) to do well at GCSE level. His wish so far is to go to an academic university, RG type.
His A Level choices are Maths, Further Maths, Physics and Chemistry, so you can see where his interests lie, although he's a solid overall performer and I would anticipate he will gain high grades in all subjects.
We are fully aware 3 subjects only count for UCAS, but, DS is a stubborn person and is unlikely to drop any A Level. He also performs very well in exams, prefers them really, and they do not stress him or cause him tension (DD is another matter....).
Given that he wishes to have a shot at gaining entry at Uni where the competition for places is high, I am wondering if he may need to start thinking already with regard to subjects.
My overall feeling is to let him settle into A Levels and see how the subjects differ, where his enjoyment then takes him, then review, but is this enough of a plan? I am aware that, as a parent, I am not very pushy. I leave him to it in the main but support his thoughts and chat when required. It's up to him at the end of the day.
For info, he very much enjoys astro physics and maths is intrinsic in his soul 
TIA.