Good evening - for the most of you at least -,
Thank you all for the great number of insightful and respectful comments.
Sending him to a US boarding school would indeed be a possibility, but, even though, I have married an American woman and live there, my feeling is that having children who only grew up in the US is not necessarily a desirable thing. In all honesty, the idea of 'American exceptionalism' and the related Americentrism frightens me, as does, for example, the political intolerance. I was not educated that way and don't wish my children to think that way. You could, of course, argue that it's the role of the parents to give them an appropriate education, which encompasses the transmission of certain values, but when they grow up in an environment that doesn't necessarily share that world view, it's difficult...
On the other hand, if - like a lot of you seem to say - it has become normal for boarders to see their parents each weekend, it will probably not be easy for my son to handle it. But, like I already said, we do as parents, of course, intend to see him once in a while and my parents would see him more often. If that will be sufficient, I do not know. I believe(d) so. Again, my wife didn't see her parents each week, at the time, and doesn't seem to be angry or unhappy about it.
Even if we would send him to a boarding school on the East Coast, we would not see him each weekend; that's just not feasible.
Overall, I imagine that it also depends on your child. And my son, for instance, seems to cope quite will with being away for a certain time, without seeing us - it doesn't mean we don't call or write him.
I talked about the subject with a friend of mine, who has sent one of his children to a Swiss boarding school and doesn't see him as often as other parents do, by cause of his very busy professional life. Nevertheless, his son seems to be quite OK with it and doesn't seem to be unhappy or to feel forgotten/neglected by his parents.
By sending our son abroad, we want him to blossom. He loves learning, he loves debating with people, he enjoys to be stimulated. We can entertain him and teach him a wide array of things, but our knowledge is not unlimited, as is our free time. My understanding and hope was, that this would be possible by sending him to certain boarding schools.
The question of deciding between A-levels or IB, is not an easy one. At the moment, we do both share the impression that our boy likes, once he finds a topic that interests him, to dive deep into it. Currently, those subjects appear to be mathematics, physics and history.
Regarding university studies, which is not really a preoccupation, I think that having studied A-levels wouldn't hinder him to join an American university (or a university in continental Europe), not sure, though, if an IB is the ideal preparation for a British university.
Furthermore, a very international school might be a good thing for him, but I don't think it has to be a conditio sine qua non. It should be somewhere in the middle.
A last point that one of you brought up, namely languages, is interesting and important. We do indeed believe that mastering several languages is a necessity. Obviously, his knowledge of the English language will only improve. It would be a pity, if he would lose his knowledge of French, which I doubt since he does speak it with me since his birth and he spends, each year, a month of his summer in Europe. His basic knowldege of Dutch, will of course remain basic. Losing his Russian would, on the other hand, really be a shame, since he speaks it quite well - far better than I do, but that's not really an achievement ;-). I have read that some schools offer Russian language courses, but I have no idea about the level of instruction or engagement.