Your DS's profile is very similar to mine. My DS is also Yr9 and choosing options. He's desperate to do history as like your DS, he loves reading about history and has a special interest in specific periods of history.
However, I have studied history myself at quite a high level and the skill set required to pass/excel in history exams is very specific. It's not just being able to regurgitate facts and write down everything you know about a topic. You have to be able to argue - with evidence - two opposing opinions. You also have to be able to do this in a very small window of time and understand exactly what the question is asking you.
I'm really not sure my DS will be able to do that. We've agreed with the school that he can take history and then we can always withdraw him if it's looking like it's going to be too much for him. (My Ds has an EHCP and is in a special unit so timetable adjustments are easier for him.)
They are still almost two years away from GCSEs though and hopefully a good school will be teaching them exam/essay techniques.
I have also been talking to him about how the study of the historical time period he's interested in can be done outside of school and can be something he does just for pleasure to try and manage his expectations.
My DS is also taking geography. Geography, I think, might be an 'easier' subject for him in that the GCSE is a combination of 'fact' and 'opinion', short one-word answers and longer ones.
I would also second other posters in that while it's good to be prepared and know what's out there, it is slightly early to be thinking about A levels. This time next year when they're half way through Yr10 I think will give us a better idea of how they're likely to perform in their GCSEs.
I would also agree that no one should be doing an A level subject they don't enjoy, even if they're good at it.