Hey,
See,since your son is in 9th standard so you must keep one fact in mind that your son's concepts are getting cleared irrespective of the books he is using.
Tell your son to focus on the books prescribed by hid  school. If he is CBSE board ,then you must make sure that he is thorough with NCERT for all the subjects. NCERT is more than enough at this mind for it will help him build up concepts and also prepare for boards.
Hope this helps.
Good luck.

Hello there, Sir.

First of all, you must provide him NCERT books of all the concerned subjects whether its CBSE, or any state board. NCERT books are more preferred than any other books. Now, as the language the subject matter of NCERT book is not easy to understand, so you may get the following books for him:

Chemistry: The most suitable for Class 9th is the Foundation Science Chemistry book authored by Dr. A.K. Singh and published by Bharti Bhawan Publication. This book follows a clear and systematic manner of subject topics and also comes with Practical Experiments and Solved Examples. The language of the book is so simple and can be easily understood by everyone.

Physics: The best reference book for Physics (Class 9th) is the Foundation Science Physics which is authored by Professor H.C. Verma who is well known research and experiments in the field of Physics. This book is also publsihed and distributed by Bharti Bhawan Publication. The topics and theories disussed in this book are more practical and less theoretical. So, it will be better for the students to understand the Practical aspects of Physics and the theories can be gathered out from the NCERT Books.

Biology: There are many reference books for Biology but none of the books are as much as good as its taught in NCERT Science Book. So, in order to prepare for Biology, the student can either learn from NCERT or use the help of NCERT Exempler. He may look for some NCERT Notes or Combined Sample Paper+Notes books like: Biology Sample Paper+Question Bank of Evergreen Publication. It contains all the subject topics in precise manner, different types of questions and answers, and 10 Sample Papers for Practice Purpose.

Mathematics: The only book which I will refer for Class 9th Student is Mathematics by R.S. Agarwal, which is published by Bharti Bhawan Publication. R.S. Agarwal is the best book ever for Mathematics preparation. All the topics and theories are clearly explained in this book. There is a step wise guidance and solution for each different type of question. Though there are much questions in the book but there is step wise guidance to solve these. So it will not create much problems in solving such questions. But, for having a better understanding, I will prefer to complete the basis of NCERT Class 9th Mathematics Theories and then come back to R.S. Agarwal. Because in examination, many questions come directly from the theory which will seem easy to do, but that will be too difficult to solve if the student has not understood NCERT Mathematics.

So, I have tried to provide the best reference books for Class 9th. In case of any more queries, feel free to mention. So, all the best for your Son's career and I wish he will achieve something good in life.