Let me share my experience with you and it might help you as I am speaking directly from what I went through.
I started my studies in ICSE Board when I was 7th and I can totally understand what your son is going through. Yes, it was very tough, it was hard to understand the marking schemes, the syllabus etc. Things were very hard for me during my 7th STD, I got very fewer marks, maybe the lowest mark throughout my school life. It did bring down my self-esteem, I felt that I could no longer study as good as I could before. I didn't know that ICSE was the toughest syllabus. I thought I was getting bad in studies.
For one whole year, I struggled to get good marks, no matter how well I studied and wrote the paper I would never score good marks. My parents got worried and they had plans of shifting me to CBSE school, after my final exam. It was my last chance in ICSE to believe in myself and get through the exam. Teachers helped me a lot, they made me understand the how marks are given, how to study systematically. Step by step I conquered my weakness, it didn't happen soon, but slow and steady from one of the weakest students I reached to top 5 students in ICSE in a year.
My suggestion would be, don't take your son yet from that school. Give him the confidence that he can, help him to conquer each subject step by step. Continuing in ICSE would benefit your son in the long run, especially in his higher studies. Putting him in SSC now might help him to score better now but later when he will have to give entrance exams for the various competitive exam it will hit him hard.
This is my experience.
Most importantly give your son the confidence that he can do it and believe in him. And of your concern for 9th Std, in ICSE schools, the basis for 10th Std starts from 8th Std itself. It is a continuation of 8th, 9th, and 10th STD. So it will only benefit him if he continues in ICSE as it will make his foundation very strong which will help him in his further studies as well. Students from SSC struggle a lot for entrance exams.