Hello Ma'am,

IGCSE (International General Certificate of Secondary Education) syllabus and education stands at an international standard whereas ISC ( Indian School Certificate) stands at a national level. IGCSE is a more creative and practical learning system whereas when it comes to ISC/CBSE, it may become a little theoretical as they oriented towards training students for enginnering and medical enteances sich as JEE and NEET. In terms of difficulty boards of education can be arranged from tough to easy as IGCSE/IB, ISC  followed by CBSE. So basically there is no need to worry, your son will find it quite easy to adapt to ISC.
Note that ,if your son is planning for higher education outside India , then sticking with IGCSE would be wise . For ISC syllabus:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://school.careers360.com/articles/isc-syllabus/amp&ved=2ahUKEwjelujY_KDuAhVWbn0KHfFjBuUQFjAAegQIAhAB&usg=AOvVaw3EVdHivr-cHeKLvy4kEFm-&ampcf=1&cshid=1610818586832

All the best