What Malermalergoni said. You need to find a school which will accept external candidates (maybe ask at his current school first) and you will need to pay fees to sit the exams there.
First steps would be too establish which exam boards he would be using - if he is going to be an external candidate you could choose for yourself, although it may depend on which exam boards your local schools offer. Bear in mind that different exam boards can cover slightly different content and have different methods of assessment and marking, so if you have any sort of choice it may be worth considering which one is best for your DS. You can usually find lists on exam board websites of which schools take external candidates for their specifications. From there you can contact the schools and discuss fees.
If he is doing subjects which include coursework it may be more complicated because many schools will accept external candidates for exams only. If they do accept you for coursework you will often have to provide your own tutor who will take responsibility for marking the coursework and sending it to the school. The school will probably prefer them to have qualifications such as a degree in the relevant subject and possibly a teaching qualification and experience of marking.
There will be a deadline for entries (depending on school and exam board) so best to start looking into it during the summer as he will need to be ready to start studying in September at the latest and you might need to submit the entry fees to the school by the January before the exams.
In terms of studying independently at home, I would say he needs to start working through the textbooks as soon as possible, make a plan for covering everything and obviously leave enough time for revision (starting around March at the latest.) DEFINITELY make use of the specification outlines, past papers, mark schemes and examiner reports (all of which can be found on exam board websites) as these will show what he will be assessed on and how it will be marked, which is very important when it comes to A levels.
Search the internet to see whether there are any blogs/websites/youtube channels that offer help for his subjects. thestudentroom can be good for exam tips and subject specific revision notes as well as the chance to talk to other people doing the same courses (a bit like mumsnet but for students).
Tutors may be a good idea if you can afford it. Although I would be careful here - in my experience tutors can be very expensive and actually offer very little help. Make sure they have recent experience of teaching the subjects your son is doing and ideally of the exam board he is sitting with. Try not to end up with someone who hasn't actually been in a classroom for ten years. That's not meant to undermine the value of retired teachers (or even younger teachers who have been home tutoring for a while) but the main thing a student needs once they reach A level is to be guided through the specification and how to pass the exam. When it comes to exams, education changes so rapidly that you really need someone who is totally up to date for them to offer you anything that is worth the money they charge. Bear in mind you can find a lot of help and information on the internet for yourself, as long as you are willing to look.
That's everything I can think of from experience with DD 