As per the existing rules he should be completing 14 years by 1st October of the academic year in which he is going to appear in class 10 exam. Your nephew has DOB as 15.10.2008 and he will not be completeing 14 years by 01.10.2021 so he may not be eligible for appearing in the exam during March/April 2022. However there is some discretion (generally up to 6 months) in limited cases with the principal of the institution and you can find about it in details from the school itself. But in this case the shortfall is almost about one year.
Knowledge is power.