In UK the state schools follow national curriculum and the exam is GCSE which is conducted for year 10 and year 11. IGCSE is the international certificate exam which (as per my knowledge) is not provided by state schools. Only the private schools provide IGCSE. I have heard GCSE and IGCSE are very different in the approach And the focus. It is possible to move from CBSE to IGCSE and vice versa. But Moving from one board to another will need some time to adjust. Because the approach is very different, more importance is given to language when compared to CBSE, the testing pattern is also different. See if it's worth the risk moving him across different boards esp. in high school after you return.