Computer Science is a good course (minus the government's incompetence at the coursework), covering a lot of stuff which is very relevant. Schools usually use Python but other languages (C#/C++/C, VB.Net, Pascal/Delphi, Java) are available from the exam board. If he knows any of these languages or any other text based one, he will find Python pretty easy, it's one of the easiest languages to get your head around. IT is the next best fit if CS if not offered.
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.