There is quite a lot of work in each GCSE so it's always advisable to plan for that and make sure you can cope with the workload. In the vast majority ofschools GCSE Maths, English Language and Science are compulsory and the other subjects you will be entered for will be options that you choose from a range offered by the school (in some establishments a language is also compulsory and in others, Religious Studies too). All in all it's worth taking a lot of time in Yr 9 identifying those which you have to take, and then finding out as much as you can about the others before choosing your options.
You can’t take exams at two different centres in the same ‘series’ (. summer or January). If the timetable permits, you could ask the school to enter you for extra exams, though you might have to pay. Ask your exams officer – they’re the experts and know you son’s situation.
It looks like you’ve read the page on science GCSEs ( http:///science-gcse/ ) so you know the structure. Because of the new flexibility that I describe on that page, the school can organise science in different ways. There are only three exams for triple science. They can be taken together in year 11, or split between yr 10 and 11. Biology in year 10 and the others in yr 11. Or, the school could go down the route of Core science in yr10 and additional plus further addition in year 11.