Hello,

Your nephew can have the advantage of choosing the Subjects or Streams only after the completion of Intermediate. Till class 11th and 12th, the syllabus for class 11 has already been prescribed by the CBSE. You will have to study according to that Only. You will have to study all the Subjects prescribed by the CBSE. Option to choose subjects is not available till your nephew pass class 12th.

For class 11th CBSE has prescribed this syllabus

1. CBSE class 11 Accountancy

2. CBSE Class 11 Business Studies

3. CBSE Class 11 Economics

4. CBSE Class 11 English

5. CBSE Class 11 Hindi

6. CBSE Class 11 Mathematics

7. CBSE Class 11 Informatics Practices

Yes, If you want then you can take Mathematics as optional subject. It totally depends upon you or on the CBSE school where your nephew is planning to take admission into.

To know more and to get detailed syllabus of CBSE class 11

https://www.google.com/amp/s/school.careers360.com/articles/cbse-class-11-commerce-syllabus/amp

Good Luck.