Maths, english and science are compulsary at GCSE so I reckon these 3 subjects are his 'optional' subjects that he took on top, and he's asking whether to take IB or A-Levels at Sixth Form.
I'm an A-Level student in Y13, soon to sit my maths, physics and geography exams. If OP has any questions about A-Levels, I can answer them. I've got a mate doing IB and the curriculum is much broader. On a CV, that
may be better; however if you're naturally better at STEM than writing - and vice versa - then A-Levels will be better as it's just 3 chosen subjects. If you intend on going to university, 3 A-Levels are accepted just as well as IB in the UK. Choose subjects that you're good at and you enjoy as it'll be easier to do better than forcing yourself to study some litertaure that you (I
) hate. A-Levels are much harder than GCSE in terms of content difficulty and volume, so although you may find GCSEs a breeze right now, it doesn't mean you'll find those same subjects easy later on. Regardless, you've got plenty of time to decide. Just do well in your GCSEs next year so you're A-Level/IB subject choices aren't limited.