A serious consideration will be whether your son is competent in german or french as it will be compulsory to do a foreign language in the IB and to be at the level of others who have most likely been in the IB through the MYP.
The difference in approach is significant and there is no time to ease into the IB if you speak to people whi have done the program academically it is the hardest program around and uni is easy afterwards. Remember everything is assessed from day one which is why the transition will be that he will be expected to fully with it from day one.
While you may think it is nothing more than just a change in school the reality is the difference in expectations is huge. My recommendation if he wants to do then make the transition in the MYP years otherwise it could be a step too far
Have you considered another option, which is to leave him in UK to do A'Levels, staying with family or friends, or go to boarding schools. There are several State boarding schools, where you only be for boarding, and not for education- some perhaps near family and friends, and near an airport with cheap flights to and from Switzerland for regular visits both ways.
Whatever you decide, find a good tutor and start intensive German lessons asap.
First of all, your son will in all probability be leaving an English school to go to an international school. There will be a difference in expectations on him, culturally he will find it different from what he is used to and, coming into the Diploma program (unless the ISBasel accepts a large cohort) he will initially be on the outside looking in.
Moving from GCSEs to IB is always difficult. GCSEs are not a particularly demanding program (it would have been preferable if he was at least studying IGCSEs) and inadequately prepare students for A Levels, never mind the IB. This is why a number of schools enhance the GCSE program with their own, rather more rigorous material.
Assuming your son is academically able and obtains a place at an IB school, he will then have to decide which subjects he wishes to study. The demands of the IB are as follows:
Compulsory: World Literature Essay, Theory of Knowledge Essay, Extended Essay, CAS Programme, Group 4 project (unless studying Environmental Systems)
Optional Choices (but still a requirement)
3 x Higher Level subjects (HL)
3 x Standard Level subjects (SL)
The subjects in the IB are divided into groups. Each subject is worth 7 points, for a total of 42 points plus 3 bonus points from his EE and ToK essays. It is possible to obtain more but difficult as it means studying for an additional subject. Depending upon what the school he is going to offers, these may be his possible choices:
Talk to your son and have an idea of what he wishes to study and where he wishes to study it. Each country has its own requirements.
Switzerland (except EHL and Webster): Maths (Group 5) must be at Standard Level or Higher Level. A science (Biology, Chemistry or Physics) must be at Higher Level.
Group 1 will be his language choice (essentially his mother tongue, although it is no longer an IB requirement). This will probably be English Literature HL
Group 2 is his second language. This can be at Ab Initio level if he has not studied the language previously for more than one year (although only at SL) otherwise it has to be a language "B" which can be either HL or SL. I would recommend whichever language he studies for GCSE.
Group 3 is the Humanities - History, Geography, Economics, Business and Management. All these can be studied at HL or SL.
Group 4 is the sciences - Physics, Chemistry, Biology, Environmental Sciences and Societies (SL only), Sports Science (if offered).
Group 5 - Mathematics HL, Mathematics SL, Mathematical Studies (SL - although not accepted in Swiss universities except EHL and Webster), Further Mathematics HL (rarely offered anywhere).
Group 6 - Any one of the above or something from Art, Theatre Studies or whatever the school offers.
Here are the caveats:
A Higher Level course is much more difficult than an A Level course. Mathematics HL, for instance, is more difficult than A Level Further Mathematics.
Is your son and all rounder or a specialist? I would not recommend him studying for any subject at HL unless he can easily obtain an A* at GCSE. If he is a specialist, then stick with A Levels.
The subject choices are important - the program has to be challenging but one also has to keep an eye on the point score. Universities value some subjects higher than others, and two or more of weaker subjects will mean that he can kiss goodbye to a place at a Russell Group university, irrespective of his point count.
To be brutally honest, either move now to an international school or leave him to do A Levels. He will find it much easier to get into a UK university with A Levels than with an IB. The IB equivalent of A*AA at A2 is 38 points, and getting the A*AA is much easier than getting 38 points.
BTW many students in UK currently do NOT study a foreign language to GCSE, as it is no longer compulsory past Year 9.
To OP, does your son study a language for GCSE, and which? The level is very low though (I taught French and German to GCSE and A'Level in UK for many many years...).
For Economics, he has to take Maths HL and obtain a minimum of a 6, probably along with History at HL (although not necessarily so). The workload for these subjects is going to be quite large.
However many students do take this route and manage, so it should not be too onerous.
Being on the outside looking in refers to the fact that in a number of international schools the majority of students who are studying at the VIth form level have known each other for a number of years. Therefore any new student will, for a period of time, be on the outside of these groups looking in. Students do get integrated, but it can take time and it is not always guaranteed. Each year we have a couple of kids leave because they did not fit in. It happens.
Secondly, the choice of subject is essential for university applications.
> Easy route: son knows where he wants to go, the subjects are chosen accordingly and plan B is thought through right from the start.
> Hard route: son doesn't know and must keep options open. A couple of standard flexible mixes of subjects will be discussed with the school according to son's abilities. This restricts freedom of choice immensely, though.
I don't have updated info as this is old stuff to me, but make sure you discuss it through very early in your moving plans with the school. Their interest is to have high passing rate for PR purposes (and out of professionalism too, I guess) so if they warn you about something, they have a good reason to do so.