@Aramox, yes, very familiar! Nearly 14 year old ds here, also at a grammar. Does the bare minimum, in bottom sets at school but not struggling. He gets what he considers to be reasonable marks in the context of his sets with very little effort. But he is also ADHD, so doesn't find concentrating easy.
I really don't think extra revision/reading around the subject is at all normal though! I also have a year 11 dd who's likely to get all 8/9s with maybe one or two 7s this summer. She worked hard for exams and put full effort into assessments. She wasn't hugely interested in any of the content though to be honest, and would never have done extra work by herself or read around.
I don't think GCSEs are that interesting to be honest - partly because in some subjects where the content is interesting (eg History) they have to rush through it because the syllabus is so huge. Also, there's a big focus on exam technique, how to get the marks, rather than interest in the content matter.
Also - in the case of my dd, I'm not sure she's academically-orientated, despite being very able. She's a people person. She said to me the other day that she can't imagine doing a three year degree in an academic subject so she wants to do something vocational like occupational therapy. I think that's fine!
Ds can get very interested in topics - he had to watch some videos about capital punishment for RS and said he found them 'compelling' but on the whole I imagine him doing something more active and less studious in the future.
So - it may just be to do with the GCSEs/school set up, which is boring. Or it may be that your ds isn't an academic type. Or it may just be that he's a lazy, immature, year 9 boy like mine! I wouldn't worry too much....