I'm a current year 11 and I do computer science. There is a lot of theory involved and the latest controlled assessment where you had to code a program based on the given scenario was just cancelled due to a leak of the solution. The coding aspect can be very difficult for someone who only has a basic level of coding, and the schools usually only offer python coding.
Other than computer science, I'd wouldn't really say there was any other subjects specific to computer coding.
Hope this is helpful
Maths is critical, and while it's a compulsory subject he needs to be good at it to take Computer Science to A Level or University. Some schools require good maths skills as a requirement to do Computer Science.
Triple Science, but particularly Physics as it includes some electronic principles (not a great deal of use, but can help in the Computer Systems section of CS), but more importantly lots of use of formulas and maths skills, which transfer across to the use of algorithms.
Other than that, if your school offers Systems and Control (either the GCSE or the Cambridge National), that is a course that ties in very well with CS (it has programming itself, plus system design and more useful electronic theory). Some parts even directly cross over with each other, making them very easy as I had covered them already.